Russel Wallace : Alfred Russell Wallace (sic)
This is the more remarkable because it is based upon facts of Nature which are not only universally admitted, but are well known at least verbally to every one who takes the slightest interest in living things; and, further, this misunderstanding is not confined to the ignorant or unscientific alone, but prevails also among the educated classes, and is found even in our highest seats of learning among the teachers and highly placed professors of various departments of Biology.
It is not my intention here to combat these strange misapprehensions in detail, but it is my hope--if I can succeed in placing the subject before my readers in all its simplicity--while yet fully emphasising its underlying complexity and grandeur--to satisfy them that the theory of Darwin is the only one that is in accordance with Nature herself--with her most obvious facts and phenomena, with the broad and fundamental laws which are in action now, and have been so throughout all past ages--and that any less far-reaching and less fundamental theories are altogether inadequate and unmeaning.
Why Darwinism is Simple and Intelligible.
Some readers may be surprised at the statement that the theory of Darwinism is essentially simple, and easily understood. Why is meant is, that it needs no special training to understand it; no laboratory work is required, no knowledge of anatomy or physiology. The facts appealed to are, throughout, the facts of external nature, which every one has (or ought to have) the opportunity of observing for himself. And the more important principles arising out of these facts are also of the most simple and obvious nature, so much so that the objection is often made that they are self-evident truisms.
It is perhaps for the very reason that Darwinism is so simple that it is so persistently misunderstood. Those who have gone [[p. 412]] through a long course of study in the anatomy, physiology, or embryology of animals or plants, very naturally think that a theory which can dispense with their work (though it is often strikingly supported thereby) cannot be of much value; while it often happens that those who have the most extensive knowledge of these departments of Biology do not possess the special type of faculty that is essential for a full grasp of the great masses of fact which underlie and are essential to the actual working of the principle of evolution through Natural Selection.
All this, and much more, will, I hope, be made clear as we proceed with the examination of our subject.
Numbers, Variety, and Intermingling of Life-forms.
The first great group of facts to which I must refer is that of the vast numbers, the astounding variety, and the universal intermingling of the species or forms under which life is manifested on the earth.
Taking first the vegetable kingdom, as that upon which the very existence of animal life depends, and limiting ourselves to the higher, or flowering plants, as those best known, and which from our point of view are alone of importance, we find that so comparatively poor a flora as that of the British Isles comprises about 1,800 species (counting only the species and sub-species in Sir J. Hooker's British Flora), but probably over 2,000 on the estimate of most local botanists. Europe has about 9,000 species, while in the whole world there are now about 136,000 described species, but perhaps nearly twice that number, if the whole area were as well explored botanically as Europe.
Thus, while Britain in proportion to its area seems to be much richer than Europe, the latter seems much poorer than the world, as it really is on account of the superabounding luxuriance of the tropics and the warmer regions, in vegetable life.
And though Europe as a whole seems poorer than Britain, yet many equal areas in Europe are much richer. Thus the European Alps from Savoy to Styria, with about the same area as Britain, has more than double the number of species. In the warmer regions of the world there is still more abundance of species--the limited Cape district of South Africa, in little more than half the area of Britain, has about 4,500 species of flowering plants.
This last is held by some botanists to be the richest plant-area in the world; but I believe when the Equatorial regions [[p. 413]] come to be as well explored by resident botanists, districts will be found which are very much richer.
The large number of distinct species that live together in small areas is very well shown by the following table from the personal observation of one of our best British botanists, the late H. C. Watson, and therefore quite trustworthy as to comparative numbers:--
We have here a most striking example of the close intermingling of the species, nearly half those of a whole county being found in one square mile of that county, while ten square miles has almost as many species as sixty square miles.
I will just mention two other examples given me by the resident clergymen of two parishes. Dorsetshire possesses 1,010 species of flowering plants, and the small parish of Edmonsham has 640 species, though only 1/340 the area. Lincolnshire, a very large and flat county, but with a very diversified soil, has 1,200 species, while the parish of Cadney, 1/920 its extent, has 720 species.
Very small areas are also often unexpectedly productive. In Cadney parish, a plot of sixteen feet square produced 38 species, while Darwin records a patch of turf 4ft. by 3ft. in which he found 20 species. In insect and bird life very similar facts have been noticed, though few observers have recorded them; and we thus see how complex must be the adaptations of the various species which enable them to live thus intermingled, each in its special station and due proportion to the rest.
This special adaptation is well shown in the case of those rare species which only exist in one or two limited spots. Such are the lovely Gentiana verna, found only in a few square miles of wild moorland in Upper Teesdale, in Yorkshire; the Spring Snowflake (Lencojum vernum) in one small thicket in West Dorset, and several confined to single mountains in the highlands of Scotland or Wales. Here the conditions seem almost identical in other adjacent areas, yet these particular plants do not find it possible to live elsewhere.
Before leaving this part of my subject, which might be greatly extended, I must call attention to the somewhat close agreement in the variety of interrelated insects and plants in the same areas. The larvæ of Lepidoptera, commonly known as caterpillars, almost all feed upon the foliage of living plants. Many [[p. 414]] plants support several species of caterpillars, while some of the commoner caterpillars will feed on several different though usually allied plants. Now it is very suggestive that the number of species of Lepidoptera and of flowering plants is very nearly the same--somewhere about 2,000. Beetles, however, are much more numerous (3,260 species), but these feed largely on decaying vegetable and animal matter, while many devour other insects.
Perhaps even more important than the numbers of the species of plants and animals and of their existing so constantly intermingled in the same area, is the fact that a very large proportion of the whole are what may be termed common or widely distributed species. Perhaps one-half of the known species are in this class, and thus the individuals in each species, though often local or thinly scattered over the country, yet exist in millions or thousands of millions. Many of our best known plants, birds, and insects, range over a large part of Europe and North Africa, while several extend all across Northern Asia to Manchuria and Japan. They are what Darwin has termed dominant species, and it is these which have always given rise to new forms when changed conditions required them. This leads us to the next branch of our subject.
The Powers of Increase of Plants and Animals.
One of the most important factors in the development of the world of life, from its first appearance to the present time, has been the power of increase possessed by every species of organism. This is best shown in plants, and in some of the lower animals, in which the possible increase of a single pair is not four- or five-fold, as in some of the higher animals, but usually many hundred- and often many thousand-fold.
A fair-sized oak tree will produce yearly at least a hundred thousand acorns, and the tree lives several centuries. The botanist Kerner states that a common British weed (Sisymbrium Sophia) often has three-quarters of a million seeds; and that if all these grew to maturity for only three years, the whole of the land-surface of the globe would not hold them all!2 A tall spike of foxglove, loaded with its seed-vessels full of minute seeds, has probably as many, as have some of the orchids and many other plants, so that the powers of increase of the vegetable world seem to us wholly beyond what can possibly be needed. But there are certain considerations which lead us to conclude that this is not so; one being that when at rare [[p. 415]] intervals very large areas are devastated by fire, by flood, or by volcanic eruption, it is of great importance that the lands so rendered bare should be covered again with vegetation as quickly as possible, and thus avoid the extinction of numerous species of both plants and animals.
Turning to the animal kingdom we still find the reproductive powers always ample and often enormous. The slowest breeding of all is the elephant, which is supposed to rear one young one every ten years; but as it lives to more than one hundred years, Darwin calculates that in 750 years (a few moments only in the geological history of the earth) each pair would, if only all their offspring lived and bred, produce nineteen millions of elephants.
The smaller mammals and most birds increase much more rapidly, as many of these produce two or more families every year. The rabbit is one of the most rapid breeders, and Mr. Kearton calculates that, under the most favourable conditions, a single pair might easily increase to a million in four or five years. Australia, being so favourable in climate, vegetation, and absence of enemies, they have so multiplied as to become a nuisance and almost a danger, and though their introduction was very easy, all our powers of destruction seem, so far, impotent against them.
It is interesting to note that the numbers of many of the species of animals bear little or no relation to their rate of increase, the slowest rate being amply sufficient to keep up the greatest population when the conditions of life are otherwise favourable. Two well-known cases are those of the American bison and the Passenger pigeon of the same country. In the eighteenth century the bison ranged over almost the whole of temperate North America, being abundant in Pennsylvania and Kentucky, sometimes reaching the east coasts; while within the memory of living persons it occurred all over the western plains in vast herds of 10,000 or 20,000 each. The Passenger pigeon lays only two eggs, but seldom rears more than one chick in each nest. The quantity of these birds was formerly enormous. In the early part of the nineteenth century the ornithologist Wilson gave a wonderful account of them. One of their breeding places in Kentucky was a tract of forest forty miles long and several miles wide, and in this space (in April and May) almost every tree was crowded with nests, both on the higher and lower branches. The parent birds daily roamed the country to a distance of nearly one hundred miles for their food and that of their young. Wilson describes one of these great flocks on its daily excursions as being more than a [[p. 416]] mile wide and of very great depth, in the air far above gunshot. They flew with immense speed--50-60 miles an hour--and the stream went on for several hours before it diminished in density, and some hours more in straggling parties. He calculated, roughly, that this one flock, going to and from one breeding place, contained about two thousand millions of birds, and this was only one out of many such aggregations known at that time in the United States.
Now, the spread of cultivation and the clearing away of the forests have entirely destroyed this most striking illustration of what Darwin termed a dominant species; which, though so apparently defenceless, with so many enemies, and with a very slow rate of increase, was yet able to maintain such an enormous population.
An equally remarkable but somewhat different phenomenon is that of the curious little Lemming of the north of Europe (something like a very short-tailed vole) which at intervals of ten or fifteen years, under some unknown favourable conditions, increases so enormously in the Arctic and sub-Arctic mountains of Scandinavia, as to overrun the adjacent parts of the country, always migrating southward and spreading out east and west towards the sea. The migrating hordes march on slowly, increasing as they go, devouring the country like a flight of locusts, sometimes continuing their march for from one to three years, but always going on, and never returning, crossing over lakes, climbing over houses, eating their way through haystacks, till they ultimately disappear in the ocean. The whole great army thus melts away, the remnants being everywhere cleared off by the multitude of enemies attracted to the feast. A somewhat similar picture of the sudden increase of swarms of rodents, is given by Mr. Hudson in that wonderful chapter of his fascinating Naturalist in La Plata entitled "A Wave of Life."
Insects show the same phenomena, but they generally possess a still higher rate of increase. The queen bee lays from two to three thousand eggs, and many moths are equally productive. But one of the most striking cases is that of the flesh-fly, which lays twenty thousand eggs and these go through their complete transformation of grub and pupa to perfect fly in two weeks. Linnæus calculated that the progeny of three of these flies would devour a dead horse as quickly as could a lion.
These overwhelming powers and possibilities of increase throughout the whole realm of Nature must never be forgotten; for they serve to explain many of the phenomena of the world of life. They afford as it were the material and the motive power which has led to the interpenetration of so many widely [[p. 417]] different types, all securing their food and that of their offspring in the same localities, yet in many different ways, so as not to interfere with each other--in the earth, the water, or the air, in field or forest, on marshes, or dry uplands, or sandy wastes. Thus, in the course of ages, every vacant place in Nature has come to be occupied, and immediately one type dies out others similarly or divergently modified are ever ready to take its place.
Birds afford many illustrations of this specialisation. Our Tree-creeper has a slender curved bill and very strong feet and toes, adapted to run up the trunks and branches of trees and pick out the small insects that hide in the crevices of the bark. In South America there is a quite distinct family, the Dendrocolaptidæ, specialised for a similar purpose. A still more numerous and more highly specialised family are the Woodpeckers, whose chisel-like beaks, extremely hard skulls, and elastic, very muscular necks, enable them to cut away the solid wood of trees so as to reach the various kinds of large grubs and other insects which make their burrows there. Their very long and extensile tongue with a horny barbed tip enables them to pierce and draw out the defenceless insect. These three families have all been modified to obtain insects from trees, yet are not closely related to each other.
Inheritance with Variation.
We now pass on to consider two combined groups of facts which are of the most vital importance to a comprehension of Darwinism--inheritance and variation.
These are often termed "laws"; but, taken in their generality, they are the commonest and best known of all the facts pertaining to living things, and it is rather the mode and extent of their occurrence than the occurrence itself which are subject to special laws. Our ignorance of the fundamental causes of these two phenomena is often adduced as a difficulty in the way of the Darwinian theory, but it has really nothing to do with it, as all we require to know are the facts. The facts alone, not their causes, are the agents in bringing about Natural Selection.
The great fact of inheritance--of each species producing its like--is so common, so universal even, that it is only the deviations from it that surprise us. That offspring should be like their parents seems to us quite natural and to require no explanation; it is only when there is a considerable difference that we begin to ask the reason why.
Inheritance, therefore, is a universal fact in the world of life-- [[p. 418]] but it is not absolute and nobody expects it to be absolute. Never do we see a son absolutely like his father--even so like as twins often are, and they are never absolutely alike. But in all well-marked species of plants or animals the offspring are so much like the parents as never to be mistaken for other species. Yet variation always exists; it is really very considerable even when small numbers of individuals are examined, and it follows certain laws of a general nature, which are of great importance to a comprehension of Darwinism, and to these laws we must therefore devote some attention.
The first great law is that the frequency of a certain amount
of variation of any character, in any species of animal or plant, depends (in any inverse ratio) upon the amount of its departure from the mean value. It may be accurately represented in a diagram by what is termed by mathematicians the Curve of Error, but perhaps more intelligibly as the Curve of Variation.
The above diagram shows this curve, obtained from the measurements of 2,600 men, taken at random. The horizontal scale gives the heights in feet and inches, while the figures are so arranged as to show the number of men at the height on the scale in a line with their inner margins.
[[p. 419]] Beginning on the left we see that only one man was so short as 4ft. 8in., about twelve were 5ft. high; while fifty were 5ft. 4in.; and above this height the numbers increase rapidly till we find 150 at 5ft. 7in. and 160 at 5ft. 8in. From this point the numbers of each successive inch of height decrease, just as they had increased up to that height, there being 150 at 5ft. 9in., only fifty at 6ft., less than ten at 6ft. 6in., and only one who reaches 6ft. 8in. The point where the numbers reach a maximum show us the mean height of the whole of the men measured; and as there were numerically more of this particular height than at any other, and also more at each half-inch or inch below or above it than at any lesser or greater height, we are almost sure to find that those about the mean height form the majority in every miscellaneous assemblage of people, even when only a few hundreds in number.
With regard to the extremes of stature there is less certainty, just because they are so rare. Among even a hundred people you might chance to find a giant or a dwarf, but if a million were measured in Europe or America we should almost certainly find some as tall as 7ft., and others as short as 4ft. 4in. By adding together the whole of the figures on the above diagram we obtain the total of the 2,600 men measured, each figure showing the number of men in a range of half an inch above and below the height on the scale.
Now these diagrams apply not to man only, but serve to show very closely what occurs with almost all the commoner animals and plants in a state of nature. This was long doubted or denied, simply because no one had taken the trouble to collect and measure large numbers of individuals of the same species. Even Darwin himself did not realise how much and how universally most wild species vary. Hence he used misleading expressions, as--"if they vary: for without variation Natural Selection can do nothing." And again--"A variety must, perhaps after a long interval, vary or present individual differences of the same favourable nature as before." These expressions (used in the Origin of Species) have been seized upon and exaggerated so as to present quite a false picture of the facts of variability in a state of nature.
As this is one of the most important series of facts on which Natural Selection depends, I must call your special attention to two other diagrams which show the actual variations among even a very few individuals of some common birds, and these will also serve to show what has now been proved to be the case with all animals and still more remarkably with plants.
This diagram shows the amount of variation among forty [[p. 420]] specimens of the common North American bird, the Redwing. These forty birds were all full-grown males, and were obtained at the same place and on the same day, so that all differences due to age, sex, season, or locality are excluded.
Each bird is represented by a dot, and the amount of variation of four distinct parts is shown. Each group of dots must be considered separately as showing a variation in proportion to that of the mean dimensions of that part. The scale of each group
or part is therefore different, as shown by the bill and the total length, which could not be here given in the true proportions.
The important thing to notice here is that in all the different parts the amount of variation in proportion to the mean dimensions of that part is very large, varying from 1/8 in the length of wing to 1/5 in the length of the tail. Of course, with hundreds or thousands of specimens the extremes of variation would be much greater, while the great irregularity in the grouping of the dots would be diminished, and with higher and higher numbers a tolerably [[p. 421]] uniform curve would at length emerge of the same character as that of our first diagram.
Another very important feature is the diversity of grouping of the dots in each case, showing that each part varies to a considerable extent independently of the other parts. This is well shown in the case of the wing and tail of these birds, the former showing a less range of variation and more regularity in the grouping on each side of the mean value. This is what we might expect, since the exact adjustment of the wing for flight is probably more important than an equally exact adjustment of the tail. The very large amount of variation in the length of the bill is a rather unexpected feature, and one that is very liable to be overlooked. The bird's bill is, however, of immense importance to it, serving all the various purposes for which mammals use their fore feet, teeth, and lips. A considerable amount of constantly occurring variability is therefore of high value as affording the material for more and more exact adjustment whenever adverse changes in the environment render such adjustment necessary.
I will now give one example of a very instructive form of diagram in which we are able to show the actual dimensions of the various parts of small animals within the limits of a page, and thus obtain more exact knowledge of the kind and degree of variability that exists in all the commoner species of animals than we could by having the specimens before us. Hundreds of collectors and naturalists had continually examined large numbers of specimens of the same species without ever noticing how much they varied from each other in every part and organ; and so constantly were these differences overlooked, that when Darwin appealed to them in support of his theory, they denied their existence. Darwin himself was not aware of the extent and universality of this variability till Mr. J. A. Allen published exact measurements so many hundreds of specimens of common American birds in 1871.
The diagram here given (see page 423) is constructed from Mr. Allen's measurements, and serves to illustrate the value of the materials he has given us. It has been chosen because the bird measured is of small size, and is thus suitable for presentation here, while the graduated proportions of the six important parts of the bird which are compared enable them to be given on the same page without overlapping.
The bird is one of the commonest in North America, where it is called the Bob-o-link, or Rice-bird. It is allied to our Yellow-hammers and Buntings and to the Ortolan of Europe. Twenty specimens were measured, as shown by the vertical [[p. 422]] rows of dots numbered 1 to 20. The specimens are arranged in the order of their size, No. 1 having the shortest and No. 20 the longest body. The upper rows of dots shows this increasing length as measured from the bottom line of the diagram, the mean length of the body of the twenty birds being 4 ½ inches. It will be seen that in none of the other parts here shown does the variation agree at all closely with that of the body length. The longest-winged bird, No. 5, has a quite short body. In No. 6 and No. 11 the tail is longer than in the largest bird, No. 20. The longest tarsus is in No. 15, the longest middle toes in Nos. 7, 9, 15, and 18--all being about equal. The outer toe is decidedly longest in No. 15; the hind toe in Nos. 7, 15, and 18 is equal and longest.
Here again we have a wonderful diversity of proportionate variation, in the external parts of one bird among only twenty specimens taken at random--a fact of the highest importance in its relation to our problem and yet one which is either positively denied, or treated by almost all our critics as if it had never been investigated.
In speaking of the results of his very important work, Mr. Allen says:--"The facts of the case show that variation of from 15 to 20 per cent. in general size, and an equal degree of variation in the relative size of different parts, may be ordinarily expected among specimens of the same species and sex taken at the same locality, while in some cases the variation is even greater than this." He further points out that each part varies to a considerable extent independently of the other parts; so that when the size varies, the proportions of all the parts vary, often to a much greater amount. The wing and tail, for example, besides varying in length, vary in the proportionate length of each feather, and this causes their outline to vary considerably in shape. The bill also varies in length, width, depth, and curvature. The tarsus also varies in length, as does each toe separately and independently; and all this not to a minute degree requiring very careful measurement to detect, but to an amount easy to be seen without any measurement, as it averages one-sixth of the whole size, and often reaches one-fourth.
We must also remember that colour and markings, which so often constitute the chief characters by which species are distinguished, are equally, if not more, variable. Mr. Allen tells us that the difference in intensity of colour between the extremes of a series of fifty or one hundred specimens of any species, collected at the same locality and the same season, is often as great as occurs between truly distinct species. The markings also, whether spots, stripes, or bands, vary to an equal [[p. 423]] extent, but these cannot, of course, be represented either in tables or diagrams.
Now the diagrams here given are only samples of an extensive body of similar facts available either in diagrams or in tables
of measurements. I myself gave sixteen such diagrams in my Darwinism, and since that first appeared far more extensive comparisons have been made, more than a thousand individuals having been, in some cases, accurately measured. In every case without exception a large amount of variation has been [[p. 424]] found to exist in every part and organ, and almost every student of the subject has re-observed the interesting and suggestive fact that the different parts of the individuals measured always exhibited a large amount of independent variability.
There is therefore now ample proof that the phenomena of variation here shown are such as are present throughout the whole realm of Nature, while it has been found that in the lower and less specialised forms the variation is very much greater than among the higher.
The Law of Natural Selection.
The various phenomena we have now seen to form essential features of every part of the world of life--its enormous extent and diversity of form, structure, and habits--the vast population of every species, especially of those which we term common--the great number and variety of the species which live together on the same area, each one more or less dependent on all the others, either as competitors for food or in escaping from enemies or from the elements, thus leading to a constant or intermittent "struggle for existence" of extreme severity--the almost incalculable powers of increase of every species, so that no vacant spaces can occur which are not very quickly taken possession of by the ever-flowing tide of new life--and, lastly, the marvellous facts of inheritance with variation, in their combined effects lead us inevitably to what Darwin termed Natural Selection, or the preservation of favoured races in the struggle for life. Herbert Spencer suggested the term "survival of the fittest," as more closely representing what actually occurs; and it is undoubtedly this survival, by extermination of the unfit, combined with universally present variation, which brings about that marvellous adaptation to the ever-varying environment, which is an essential feature of every living creature which survives to produce offspring.
All the evidence at our command goes to prove that species remain unchanged for long periods, certainly for many thousand years. Some few have no doubt become extinct during historical times, but that has always been due to human agency. Since the last Glacial Epoch, however--a period of time estimated for Europe as not more than 80,000 years, and in many districts much less, the remains we find of plants and animals are all of species still living, though their distribution has become somewhat different from what it formerly was.
There is thus a wonderful stability of the forms of life under all the vicissitudes of climate and of general environment which have occurred during the last ten or twenty thousand years or perhaps more; and this alone demonstrates the complete though [[p. 425]] generalised adaptation of every species to what may be termed a constantly fluctuating environment--but fluctuating periodically and within defined limits, the average conditions during each century, for example, being almost identical.
Now this continuous preservation of the same complex aggregate of life-forms is evidently due to the series of facts here adduced--enormous powers of multiplication, combined with a large amount of variation of every part, every organ, every faculty, in each generation. For, let us consider what happens in one of those severe winters which occur only a few times or only once in a century, when even in our favoured climate the ground is frozen two or three feet deep, or buried for a month together in a vast snow-field. An enormous destruction from cold or absence of food then occurs among many of our smaller birds and mammals, but so wonderfully are they adapted to support life under these adverse conditions, that some (probably many in actual numbers), always escape, either from capacity to resist cold or from greater ability to procure food or from having found shelter--and these "fittest" that survive rapidly increase during the succeeding summer, and their offspring for the most part inheriting their "fitness" quickly stock the country with well-adapted forms.
Nature of Adaptations.
Before going further it will be well to give a few facts illustrating the nature and amount of the adaptations that are present everywhere, and on which the whole existence of living things depends.
It is an obvious fact that animal life of every kind depends on the vegetable world for its very existence, and it can also be easily perceived that it is on the overwhelming variety of plants that the corresponding variety of animals has been rendered possible.
Though so well known it may be useful to recall here Darwin's striking example of the complex relations and inter-actions of animals and plants, in which he showed that cats may determine the abundance of a plant which has no direct relation with them whatever. The common red clover is fertilised by humble-bees almost exclusively, and without these insects little or no seed is produced; field-mice destroy the nests of humble-bees and feed upon larvæ; cats feed upon the field-mice. Hence, where there are plenty of cats the field-mice are destroyed, the humble-bees then increase, the red clover is well fertilised and produces plenty of seeds. Thus without cats there would be a great scarcity both of humble-bees and of red clover.
[[p. 426]] No doubt there are hundreds of such complex relations of which we know nothing, but I will here ask your attention to a few of the broader adaptations of familiar plants and animals to each other.
Most persons, especially those who have gardens, occasionally see with horror the leaves and buds of their fruit trees devastated by various kinds of caterpillars, while everyone has noticed in certain years the oak-trees almost denuded of leaves by insect enemies. Many garden-crops, and even fields of grass, are sometimes destroyed by wire-worms and various other insect larvæ, which burrow in the soil and devour the roots. And we are apt to look upon all this defacement and destruction as we do upon epidemics in the animal world. They seem to us to be inflictions which we should be much better without, and are inclined to wonder why trees and shrubs, herbs and flowers, should not be allowed to develop in their full and beautiful luxuriance of foliage and blossoms. Yet all this destruction and disfigurement arises from a little exceptional increase of a portion of the life-world, without which some of its most beautiful features could not exist. To show how this is we need only consider what takes place in our country every spring and summer.
At that delightful season our gardens and hedgerows, our copses and woods are thronged with birds both resident and migratory, which are building their nests and rearing their young. A considerable number of these--thrushes, warblers, tits, finches, and many others--are so prolific that they have two or three, or even more families every year, so that the young birds reared annually by each pair vary from four to five up to ten or twenty or even more.
Now, when we consider that the parents of these are all common birds and must exist in our islands in numbers amounting to several millions each, we can partially realise the quantity of food required to rear say five or ten times this number of young birds from the egg up to full growth--and the whole of the food they consume consists of the various caterpillars and other insect-larvæ, with occasionally small worms and molluscs. Even the common sparrow, though one of the great army of specialised seed-eaters, feeds its young on caterpillars, this kind of food being the most digestible for young birds.
In numerous cases (both by old and recent observers) many of these birds have been closely watched while feeding their young, with very interesting results. A chiff-chaff fed its five young ones almost incessantly from morning till night. She brought small caterpillars, aphides, and flies at the rate of four times in five minutes.
[[p. 427]] A pair of blue tits, with a large family, worked for sixteen hours a day at midsummer, and it was estimated that they brought in that time about two thousand caterpillars or small grubs.
A pair of marsh tits, with a crowded nest of young ones, always went and came together--their mouths filled with small green caterpillars for their chicks.
Flycatchers sit on a dead branch near the nest, from which they catch flies, &c., in the air, and bring them to their young at intervals of from two to five minutes; while a wren brought food to its nestlings 278 times in a day, or about every three minutes for fourteen hours!
The abundance of nests in all suitable places is known to every egg-collecting schoolboy; while Mr. Kearton found nine species of birds with nests and young in a small copse in Hertfordshire, all within fifty yards of each other; and in another case three nests--a tit's, a flycatcher's, and a wood wren's--were only ten or fifteen yards apart. Yet all these found sufficient food for their young and for themselves in the immediately surrounding trees and bushes, or among the herbage below them.
And this vast destruction of insect-life goes on for months together, and the supply never seems to fail. When the parent birds leave the nest in search of food, they may be seen to fly to some adjacent bush, hop about it rapidly, and then perhaps fly off to another, where, finding what they require, they soon get a throat full of small caterpillars and return to the nest; but unless the numbers of such insects were enormous, and their development from the egg were going on day by day, week after week, and month after month, it would seem to be quite impossible for the many millions of these small birds to succeed in finding the required daily supply without fail. For if they had to go further away, or if the caterpillars required much longer searching for, the young would soon die of starvation, or of cold, or be devoured by stronger birds or small arboreal mammals during their parents' absence.
What a wonderful perfection of adjustment there must be in these little creatures, what acuteness of vision, rapidity of motion, and parental love, enabling them to keep up this constant search for food, the extreme care and watchfulness of their nests and young, on the continuance of which day by day for several weeks the very existence of those young depends.
But all this would be of no use unless the insect-tribes were so abundant, so varied, and so omnipresent as they are, and also unless vegetation was everywhere so luxuriant, and its [[p. 428]] productive power of leaves and buds so superabundant, that the destruction of a considerable proportion of it by insects rarely produced permanent injury to the individuals, much less to the species.
Here, then, we see that what we term insect-pests, when they are a little more abundant than usual in our gardens and orchards, do not exist solely for themselves as an apparently useless part of the scheme of Nature, but are and have been through the geological ages absolutely essential to the original development and continued existence of the most wonderful, delightful and beautiful of the living things around us--of our garden pets and song-birds. Without the myriad swarms of insect-life everywhere devouring the new and luxuriant vegetation, the nightingale and the lark, the red-breast, the wren, and the fairy-like tits and gold-crests would disappear for ever.
A Vision of the Earlier Life-World.
By the help of the fragmentary, though often abundant records preserved for us in the earth's crust, we are able to some extent to picture to ourselves an earlier and a still earlier world, always, as now, teeming with life, but, just in proportion as we recede into the past, with life of a somewhat lower type, and manifested through somewhat less numerous and less varied specific and family groups. The ancestral types were always more generalised--their successors in the line of descent more specialised.
Always and everywhere, these life-forms were being slowly modified (as they still are) so as to keep them in constant adaptation with the slowly changing environment. The great gaps we find in the existing series of classes, orders, and families, are well explained by the continuous increase of specialisation and of adaptation, which necessarily involves the dying out of most of the less specialised and less adapted forms. But, at the same time, numbers of the most highly specialised forms of earlier times also died out; and this, too, was a necessary result of the process of evolution. These were for the most part specialised in adaptation to local and temporary conditions of environment in an ever-advancing life-world; and when those conditions changed--when new and more powerful competitors or enemies were developed--these specialised and often huge and unwieldy forms could no longer exist. Thus it was, probably, that as birds and the larger mammalia developed the flying Pterodactyles, and the huge Atlantosaurus and Diplodocus with hosts of other reptilian forms first diminished in numbers and then became extinct.
[[p. 429]] A Common Objection Answered.
Perhaps the best way of explaining how Natural Selection actually works will be by quoting one of the common objections to it, and showing how the actual facts of Nature afford a sufficient reply. The most common of all the objections to the action of survival of the fittest in the production of new species rests upon the strange belief that variation is a rare phenomenon, that favourable variations occur singly and at long intervals, and, therefore, can have no effect in producing any important change.
As a rather recent example of this objection we may take its statement by the late Lord Salisbury in his Presidential Address to the British Association at Oxford in 1894.
After describing how the most diverse races are produced by artificial selection, he continued:--
"But in Natural Selection, who is to supply the breeder's place? Unless the crossing is properly arranged the new breed will never come into being. What is to secure that the two individuals of opposite sexes in the primeval forest, who have been both accidentally blessed with the same advantageous variation, shall meet, and transmit by inheritance, that variation to their successors? Unless this step is made good the modification will never get a start; and yet there is nothing to ensure that step but pure chance. The law of chance takes the place of the cattle-breeder or the pigeon-fancier."
Here we have it plainly set forth that advantageous variations occur singly, on rare occasions, and remote from each other; and that even when they do occur, unless by some lucky accident a male and female should accidentally find each other "in the primeval forest" nothing happens, and the "advantageous" variations are swamped in the general mass of the species supposed not to vary.
What totally false ideas of Nature such critics must have, both as to the numbers of individuals in every common, widespread, and dominant species, and as to the nature and amount of variation, to imagine that the very existence of the organic world during each period of changing conditions should have been dependent on the chance meetings of single individuals--that, having millions, even hundreds and thousands of millions to her hand, Nature should be dependent on a few scattered individuals only!
Let us recur for a moment to the numbers of individuals in most of the common species. The "Brambling"--one of our rather common autumn and winter migrants--comes to us often in immense flocks of from fifty thousand to one hundred thousand individuals each. It breeds in the Arctic regions, [[p. 430]] feeding its young on the myriads of mosquitoes and other flies that often darken the air in those regions; and in early autumn spreads southward over all Europe and a good deal of Asia, only a kind of small overflow of the migrating stream coming to us. On the Continent the numbers have been occasionally estimated. In Lorraine in 1765 about twenty thousand were killed every night for many nights in succession, and in 1865 in Luxembourg, a single flight was estimated by a French ornithologist to have numbered sixty millions of birds. What must have been the numbers spread over the whole of Europe and much of North Africa and Asia? Probably a thousand millions would be below the mark. And with these all varying, in every part and organ to the large amount shown to exist in every common species which has been measured, and with such very slow changes of the environment as we know to occur, there is hardly any conceivable modification that could be required to bring such a species into harmony with the new conditions, but such as would easily be effected in even a few generations.
It must be remembered also that Nature does not act by preserving favourable variations in such characters only as we can observe and measure, but in their results, as shown in faculties and powers of action; and as regards any such faculty or power the whole adult population of every species can always be divided into two nearly equal portions, those which possess the faculty in a greater or a less degree than the mean value. Thus, if a new enemy comes into a country, and can be avoided or escaped only by a combination of watchfulness and speed (or any other combination of characters) then the whole mass of a species will consist of those which possess the combination in a degree above the average, or below it. In countries like Europe or North America there will be many hundred millions of each sort; and we can hardly suppose that the new enemy could at once exterminate even the less adapted half. The better adapted would almost wholly escape, and would thenceforth, by producing their like, increase the adaptation of the whole species year by year till only a moderate toll could be taken by the enemy. And it must always be remembered that the enemy would not limit its attacks to one species only, but would, for a number of years, find an ample supply of food by capturing the young and the less-adapted members of perhaps a score or even a hundred species inhabiting the same country, so that there would be ample time for the better adapted portion to raise its standard year by year, owing to the regular elimination of the less fit in regard to this special enemy. The enemy would itself bring about the adaptation to the new and somewhat less favourable environment its presence had created.
[[p. 431]] Now I cannot imagine any state of things to which this reasoning will not apply. No living thing can continue to exist which is not sufficiently adapted to all the recurring phases of its environment, inorganic and organic. The completeness and generality of the adjustment is indicated, as Darwin has pointed out, by the average number of individuals in a species and the extent of the area it occupies; and it is the populous and widespread species which vary most, and which, under changed conditions, give rise to several divergent new forms, or species, each of them often adapted to fill some special place in the economy of Nature.
It follows--not as a theory but as a fact--that whenever any advantageous variation is needed, it can only consist in an increase or a decrease of some power or faculty already existing, and variations of these powers or faculties do actually occur in every dominant species, not by ones or twos as the popular critics assume, but at least by millions and in most cases by hundreds or by thousands of millions.
What midsummer madness then is this often repeated tale of the one or two superior individuals meeting by chance in the primeval forest!
Hardly less remote from the actual facts and processes of Nature which are effective in originating new species, are those modern studies termed Mutationism and Mendelism, which deal only with some of the rarer forms of variation occurring chiefly under the influence of domestication or cultivation. Yet the students of these strictly limited phenomena which have seldom been found under natural conditions sometimes claim for them an important part in the development of the entire organic world comparable with that of universal variation and unceasing elimination of the least adapted portion.
To both these theories the objection put forth by Lord Salisbury does really apply, because they deal with rarely occurring forms of variation. Herbert Spencer's objection as to the need of many coincident and co-ordinated variations in order to bring about improved powers or qualities, also applies to them with very great force, because, even the single variations being rare in Nature, combinations of co-ordinated variations would hardly ever occur. But neither objection applies to Darwinism, because the many millions of individual differences constantly present in dominant species would supply the combinations which lead to increased capacities and powers of action in ample quantity.
Protective Colours and Mimicry.
Among the innumerable subjects of interest presented by living things which are only intelligible by means of the facts and [[p. 432]] processes which I have endeavoured to set before you, are those of protective coloration among the more highly organised animals, and the endless modifications of flowers and fruit so as to benefit the former by securing cross-fertilisation, and the latter by aiding in their wider dispersal.
Most persons interested in Nature know something of the phenomena of mimicry which, since its discovery by Bates soon after the publication of the Origin of Species, has been greatly extended in its range and somewhat improved in its explanation, so that it now forms one of the most important branches of the study of insects as regards adaptation to their environment.
In every department of Nature colour is one of the most variable of all characters, and it is this variability, together with the enormous importance to all insects of concealment from or protection against their innumerable enemies, especially in tropical countries, that has enabled those minute and striking resemblances to be brought about that were long the greatest puzzle to the naturalists who had the opportunity of observing them in their native haunts.
The facts already given with regard to the universality of variation, enormous powers of multiplication, and incessant weeding out of the unfit afford a complete explanation of the phenomena of colour, in all their variety and beauty, while no other adequate explanation has ever been set forth, or even attempted.
In concluding this necessarily incomplete exposition of the fundamental facts which alone render Organic Evolution intelligible, I must again impress upon my readers the enormous scale upon which Nature works. It is for this purpose that I have put before them a number of figures, not easily accessible to the general reader, in order to give some idea of the superabundance of life, whether in its endless diversity of types and of specific forms, or the overwhelming quantity of the individuals comprised in all the more dominant species.
It was also necessary to show by direct evidence and illustration the universality of variation, pervading every part and organ, every form of action, every sense, instinct, and emotion; and further, that these variations are not "infinitesimal," as so often stated, not even small in proportion to the mean dimensions, but of such amounts as are easily to be seen even without measurements. Everyone knows that no two of his friend's children are so much alike that the difference cannot be at once perceived when they are together; and this is true throughout Nature, as shown by the fact that the huntsman knows each of [[p. 433]] his hounds and the shepherd each of his sheep, and recognises also their differences of faculty and character.
The enormous powers of increase, year by year, and the complex relations and interactions of all the various plants and animals that live together in each considerable area, complete the series of facts which, taken as a whole, render Natural Selection inevitable. And the reason why this great principle is not seen and recognised by every observer is, firstly, because it has not been systematically looked for, but mainly perhaps because it only occurs on a large scale under permanently changed conditions of the environment. There are, however, several cases in which it has been clearly seen at work--in the rabbits of Porto Santo, the mice upon an island in Dublin Bay, the sparrows in the Rhode Island blizzard, and the crabs in Plymouth Sound--all briefly described in my Darwinism.
Again, we must always remember that where we observe or experiment with tens or hundreds of individuals, Nature carries on her work with millions and thousands of millions; that, whereas our observations are only intermittent and for short periods, Nature acts perpetually and has so acted throughout all past geological time; and, lastly, that while we are concerned with one or two species at a time, and to a large extent ignorantly and blindly, she acts simultaneously on all living things--plants as well as animals, that occupy the same area--and always in such a way as to preserve every advantageous variation, however slight, in all those which are destined to continue the race and to become, step by step, modified into new species in strict adaptation to the new conditions which are slowly being evolved.
It is only by continually keeping in our minds all the facts of Nature which I have endeavoured to set forth that we can possibly realise and comprehend the great problems presented by the World of Life--its persistence in ever-varying but unchecked development throughout the geological ages, the exact adaptations of every species to its actual environment both inorganic and organic, and the exquisite forms of beauty and harmony in flower and fruit, in mammal and bird, in mollusc and in the infinitude of the insect-tribes, which have all been brought into existence through the unknown but supremely marvellous powers of Life in strict relation to the great law of Usefulness, which constitutes the fundamental principle of Darwinism.
Before concluding I must, however, add a few words to avoid misconception. Neither Darwinism nor any other theory in science or philosophy can give more than a secondary explanation of phenomena. Some deeper power or cause always has to be postulated. I have here claimed that the known facts, when fully examined and reasoned out, are adequate to explain the [[p. 434]] method of Organic Evolution; yet the underlying fundamental causes are, and will probably ever remain, not only unknown, but even inconceivable by us. The mysterious power we term life, which alone renders possible the production from a few of the chemical elements of such diverse fabrics as bone and skin, horn and hair, muscle and nerve, and brain cells; which from identical soil, water, and air, manufactures all the infinitely varied products of the vegetable kingdom--the thousand delicious fruits for our use and enjoyment, the endless woods and fibres, gums and oils and resins, to serve the purposes of our ever-developing arts and manufactures, will surely never be explained--as many suppose they will be, in terms of mere matter and motion.
But beyond even these marvels is the yet greater marvel of that ever-present organising and guiding power, which--to take a single example--generation after generation, and even year after year during the life of the individual, builds up anew that most wonderful congeries of organs, the bird's covering of feathers. Not only is a feather a miracle of complex structure, in every minutest part adapted for most important and even vital ends, but it may be safely stated that no two feathers on any bird are absolutely identical, varying in contour, in curvature, in rigidity, in size, by almost imperceptible gradations, so that each fulfils its special purpose. And beyond this, in the great majority of cases, these feathers are adorned with colours, which are infinitely varied, and which we can so often perceive to be of use to the individual, the sex, or the species, that we conclude all to be so. But to produce the result of well-defined and very constant colours, shades, and patterns on the outer surface of the bird, each feather has to be coloured on that portion of its surface which is not overlapped by the adjoining feathers at the time when the colour is needed, and this is invariably the case.
Every attempt to explain these phenomena--even Darwin's highly complex and difficult theory of Pangenesis--utterly breaks down; so that now, even the extreme monists, such as Haeckel, are driven to the supposition that every ultimate cell is a conscious, intelligent individual, that knows where to go and what to do, goes there and does it!
These unavailing efforts to explain the inexplicable, whether in the details of any one living thing, or in the origin of life itself, seem to me to lead us to the irresistible conclusion that beyond and above all terrestrial agencies there is some great source of energy and guidance, which in unknown ways pervades every form of organised life, and of which we ourselves are the ultimate and fore-ordained outcome.
the revised lecture, a portion of which only was delivered at the Royal
Institution on Friday, January 22nd, 1909.