[[p. 121]] While the general doctrine of evolution is nowhere more appreciated than among American naturalists, the same can hardly be said for that particular form of it which is usually known as Darwinism. For many years there has been a tendency among the younger students of nature--those who have grown up under the influence of the new ideas and the new methods of research which have prevailed since the 'Origin of Species' was published--to be dissatisfied with the doctrine of natural selection as an explanation of the forms, structures, and complex relations of organized beings. While admitting the reality and importance of Darwin's great discovery, they urge that it affords at best but a partial explanation of the facts; that it deals with only half, and that the least important half, of the problem. They maintain that the real essence of the problem is to discover the efficient causes of those "variations" which are selected, of those "fittest" modifications of structure which can alone survive; and they believe that the time has come when these great problems may be successfully attacked, and a new light thrown on the inmost recesses of nature's laboratory.

    Foremost of this school is Dr. E. D. Cope, the well-known anatomist and palæontologist, who in the work now under notice has collected together the various essays, papers, lectures, and addresses by means of which, during the last eighteen years, he has made known his views on evolution in all its bearings. We find here discussions on the relations of animals and plants, on the influence of mind in nature, and on the origin of consciousness, with others on less recondite problems, such as the causes which have brought about the specialized teeth and feet of the various orders of mammalia, and the general evidences for evolution afforded by the vertebrate animals. Some of these questions are discussed at great length, and in several distinct essays from somewhat different points of view, so [[p. 122]] that it is not always easy to get at the author's exact meaning. We will do our best, however, to give our readers a general idea of Mr. Cope's views upon some of the more interesting questions, pointing out where, in our opinion, his arguments or his conclusions are open to criticism.

    One of the largest and most important chapters is that on the Origin of Genera, which, it is alleged, is a more distinct subject from the origin of species than has been supposed. Throughout the whole essay it is tacitly assumed that generic characters are of a different nature from specific characters, and are always to be distinguished from them, and hence that "genera" are definite realities in nature quite distinct from mere groups of species. This view is directly opposed to the teaching of Darwin and to the experience of systematists, to whom the successive grades of sub-genus, genus, and sub-family are often uncertain and fluctuating; while the idea seems to have arisen from the author's greater familiarity with the skeletons than with the external structures of animals. Mr. Cope further maintains that allied genera differ from each other by single characters only, and that these characters are rarely adaptations to external needs or forces, but are due to laws of acceleration or retardation of growth which act without any reference to fitness at all. In other chapters this acceleration is said to be due to "growth force," and to be determined in its direction by use and effort.

    It is interesting to note that this leads to an origin of genera quite distinct from that postulated by Darwin, and which has seemed to most naturalists not only to be simple and probable, but to be a logical result of the theory of development. On the theory of natural selection, new species are produced when any change of conditions requires slight modifications either in color, form, or structure to keep up the harmony between the animal and its environment; or when the struggle for existence is so severe that some individuals of a species are driven to adopt a somewhat different mode of life from their fellows and become modified to suit it--as, for example, when allied terrestrial and arboreal species have acquired differently formed feet. Now, if a species which has thus struck out a new course of life becomes gradually modified into more perfect adaptation to it, and then gives rise to a set of varieties or new species each adapted to some specialized form of that new mode of life, this new group of species constitutes a new genus, and is characterized usually by some structural peculiarity--such as differences in the feet or teeth--which is the special adaptation it has needed and acquired. On this view every new genus has originated in some one modified species of an allied genus.

    Mr. Cope's view is very different from this. He believes, to use his own words, "that a new genus has been established by the transition of a number of species of a preceding genus, without necessary loss of specific characters"; and he even thinks the same process may have gone on in the transition of a large number of genera into new orders or sub-orders "by the assumption or loss of the character or characters of that to or from which they were transferred, and that without necessary loss of their generic characters." He believes that this explains the similarity of external form which is often found in corresponding species of distinct genera or distinct families, a similarity which Darwinians explain by the influence of similar conditions affecting growth or coloration.

    To make the difference of principle clear to readers not familiar with minute generic distinctions, we will take an extreme and imaginary case. Let us suppose that the two great families of the wolves and foxes (Canidæ), on the one hand, and the cats (Felidæ), on the other, were much more alike externally than they really are, differing mainly in the important character of the retractile and non-retractile claws and in the habits corresponding to this difference, and that it is required to explain how the Felidæ had been derived from the Canidæ. According to Darwin, some species of fox-like animal had been driven to climb trees in search of food, and every modification giving increased prehensile power to the claws and increased agility had been preserved by natural selection; while subsequently many modifications of this type had been developed to prey upon different kinds of animals, some of the larger ones again becoming adapted to a terrestrial life, and using their agile bodies and retractile claws to capture antelopes, deer, and other swift and wary creatures. But, according to the theory of Mr. Cope, a number of species of primeval Canidæ each acquired independently the characters of retractile claws and agile limbs, and formed the new group Felidæ, which was thus not derived from any one common ancestor, but from a number of distinct species which, through similar causes, had undergone an almost identical structural change. It must be clearly understood that this is only an illustration, since the two families referred to are too remote and too specialized to have been derived directly the one from the other; but the principle is the same as that which would apply to any two distinct but closely allied groups.

    Perhaps the most distinctive feature of the new school is the return to the teaching of Lamarck, that use and effort are the immediate causes of useful variations, and that the individuals which exhibit these are the "fittest" which survive. It is claimed that the "origin of the fittest" can be explained in no other way, and that, until the right variations are produced, no effective selection or useful survival can take place. With regard to the effects of use, Mr. Cope says:

    To all this the Darwinian would probably reply that no causal connection has been shown to exist between the feeding on flesh instead of insects, and the production of a hook or tooth to the bill. The use of the bill seizing larger animals might conceivably strengthen the muscles or even enlarge the bill itself, but could have no such effect as causing the tip of the bill to bend downwards, or a definite portion of the margin to grow out into a tooth. On the other hand, any individual variations in the form of the bill tending towards a hook or a tooth (and just such variations occur now whenever large numbers of specimens are examined) would give a slight advantage in seizing the prey, and would therefore be preserved and accumulated by natural selection. So with the tails of marine animals: the action of swimming might strengthen the muscles of the tail and lead to an increase of its size, but how it should lead to its compression and lateral expansion is by no means clear. Any such change of form would, however, undoubtedly improve it as a swimming instrument, and thus every variation in that direction would be preserved.

    An equal want of relation between the supposed cause and the result to be explained exists in the case of the rattle of the rattlesnake, where "the habit of violent vibration" is said to tend "to determine an increased amount of nutritive fluid" to the tail. This might increase the muscular power of the tail by the constant use of special muscles, but something more is required to account for the formation of the horny rattles and modified caudal vertebræ of the rattlesnake.

    Use, however, implies some organ or part to be used, the variations in which can be preserved by natural selection, and it is urged with some force that the beginnings of new structures or organs cannot be thus accounted for. For this the doctrine of effort is appealed to, as being a power adequate to direct the "growth-force" in a definite direction, and thus initiate a new organ. The power of imagination or desire acting as a stimulus to the secretions is called in to aid and extend the effects of this principle. As one of the best illustrations of the effects of effort, Mr. Cope refers to the production of horns or shields on the heads of so many animals. Toads are said to protect themselves from attack by presenting the top of the head forward and using it as a shield, and the result is seen in the more highly developed forms possessing more completely ossified crania, while some have projecting angles on the head rarely developed into short horns. The phrynosoma, or horned toad of California and Arizona, is really a sluggish lizard which has developed sharp horns on the back of its head for protection. In the case of the Ruminants, it is remarked that they have the habit of defending themselves by bending down the head with the horns forward, and this habit in the primitive hornless ancestors is supposed to have produced the horns by the combined action of the mental effort directing nutriment to that part, and the surface irritation produced by friction whenever the head was used in combat or defence. In support of this view it is pointed out that the generalized group represented by the hog, which were no doubt the genetic predecessors of the Ruminants, also throw the head down in defence in the same way. It is not, however, explained how it is that in this case tusks and not horns have been developed.

    Perhaps the most valuable and suggestive of the applications of the theory of use in modifying structure is presented in the short chapter "On the Origin of the Foot-Structures of the Ungulates." The remarkable progressive change of a four or five-toed ancestor into the one-toed horse, and the equally remarkable division of the whole group of ungulate animals into the odd-toed and even-toed divisions, are attempted to be explained by the effects of impact and use among animals which frequented hard or swampy ground respectively. On hard ground it is urged that the long middle toe would be most used and subjected to the greatest strains, and would therefore acquire both strength and development. It would then be still more exclusively used, and the extra nourishment required by it would be drawn from the adjacent less-used toes, which would accordingly diminish in size, till, after a long series of changes, the records of which are so well preserved in our American tertiary rocks, the true one toed horse was developed. In soft or swampy ground, on the other hand, the tendency would be to spread out the foot so that there were two toes on each side. The two middle toes would thus be most used and most subject to strains, and would therefore increase at the expense of the lateral toes. There would be, [[p. 123]] no doubt, an advantage in these two functional toes being of equal size so as to prevent twisting of the foot while walking, and variations tending to bring this about would be advantageous and would therefore be preserved. Thus, by a parallel series of changes in another direction adapted to a distinct set of conditions, we should arrive at the symmetrical divided hoofs of our deer and cattle. The fact that sheep and goats are specially mountain and rock loving animals may be explained by their being a later modification, since the divided hoof once formed is evidently well adapted to secure a firm footing on rugged and precipitous ground, although it could hardly have been first developed in such localities. It will be noted, however, that throughout all this series of changes the influence of natural selection would be required to seize upon and preserve each favorable variation as it occurred.

    Another interesting chapter is that on "The Evolution of the Vertebrata Progressive and Retrogressive." Many types in all orders of vertebrates present rudimentary organs, as rudimentary digits, feet, or limbs, rudimental fins, teeth, or wings. There is scarcely an organ or part which is not somewhere in a rudimental and more or less useless condition. These cases may be either persistent primitive conditions to be regarded as ancestral types which have survived to the present time, or, on the other hand, they may be the results of a process of degeneration, and therefore comparatively modern. The former interpretation was once the most generally adopted, but of late years so many proofs of degeneration have been discovered that it is seen to have played a very important part in bringing about the existing forms of animal and plant life. Mr. Cope defines degeneracy as a loss of parts without corresponding development of other parts. In determining the grade of development (or what should be termed advancement) in the organic scale, we have often to strike a balance between degeneration and progression according to the importance of the parts implicated. We regard sensibility as the highest of animal functions, and mind as the highest form of sensibility. Therefore development of organs of sensibility and mind constitutes a better claim of progress than development of stomach or of skin. The ruminating animals are said to be much superior to man in the structure of their feet, teeth, and stomach, yet we assign the higher position to the quadrumana and to man, on account of the superior complication of their brain structure.

    The earliest reptiles, the theromorpha, were more nearly allied to mammalia than any now living, so that reptiles as a whole may be said to have degenerated. They have, however, become specialized for certain modes of life, and the whole order of snakes have entirely lost the limbs, so that we have here a process of creation by degeneration on a large scale. Some lizards have also lost either the fore or hind limbs, or in some cases all the limbs, thus resembling snakes in outward form though differing from them altogether in structure. The earliest birds probably possessed a long bony tail as well as teeth, both of which they have lost. The whales and manatees among the mammalia have degenerated by the loss of their posterior limbs and the modification of their fore limbs into swimming paddles. The seals have partially degenerated from the carnivorous type, and the sloths and ant-eaters are supposed to have lost the teeth which their ancestors possessed, and thus to have suffered degeneration.

    One of the most curious speculations to be found in this volume is the tracing of plant to animal life through a progressive automatism and loss of consciousness. It is urged that all definite motion in organic beings must have been once conscious motion, and that all automatic or unconscious motions or acts were acquired and organized in states of consciousness. All evidence goes to show that actions sufficiently often repeated become automatic, and it is now believed that all the vital actions of the heart, lungs, stomach, and other organs originated in voluntary motions of some lowly organized ancestor. Mr. Cope thus applies this principle to plants:

    Whether this remarkable theory of the relations of plant and animal life be ultimately proved to be true or not, it is certainly very suggestive, and opens up before us illimitable vistas of inquiry into the obscure arcana of nature. Many such intellectual gems may be found in the volume under consideration, but they have to be dug out from a mass of unattractive ore. The great fault of the book is that no attempt has been made to systematize and weld together the material of which it is composed. Hence frequent repetitions and self-quotations, with a considerable amount of purely technical matter, quite unreadable by any one but an experienced anatomist. There is ample proof of the author's ability to write a connected work which should set forth his mature views on the great problems of evolution in an attractive as well as an instructive manner; and it is much to be regretted that the somewhat pretentious title here attached to a mere reprint of his collected papers was not reserved for such a work.