by Arnold E. Ortmann (1902)
[[p. 267]] During the last decennium Zoogeography has developed in a very peculiar direction, which, in a large part, is directly opposite to the methods introduced by Wallace. The professed aim of the latter was the creation of a zoogeographical division of the earth's surface into regions, realms and the like, the purpose of which was the subordination of the facts of animal distribution under a fixed scheme; and since it was self-evident from the beginning that the distribution of animals ought to express the physical conditions of the earth's surface, it was assumed that the proposed zoogeographical divisions correspond to the chief features of the distribution of the conditions of life.
Soon, however, it was discovered that it is impossible to give a division of the earth's surface that could claim general recognition. It is true that each of the proposed schemes was actually supported by more or less numerous instances of distribution, and that in many cases the physical factors influencing and explaining these divisions were easily understood; but there was always alongside of the supposed normal conditions a number of exceptional cases, where the actual distribution of certain animals or animal groups was directly the opposite. One of the chief causes of this fact has already been recognized and carefully studied by Wallace. It is [[p. 268]] the difference of the means of dispersal of the various groups of animals. On account of these anomalies Wallace constructed his regions chiefly for Mammals and Birds, excluding all the rest of the animal kingdom.1
This method, however, can never be satisfactory. It amounts to nothing but the creation of an arbitrary scheme which may correspond to some of the facts; but if there are any other facts that do not fit into it--as very often happens--they are simply thrown out and neglected.
But this is not all. Even the restriction of Wallace's regions to a single group of animals proved insufficient to cover all cases within this group. This is true also of all other schemes that have been proposed by other writers for the same or other smaller groups. In every single instance there were exceptions to the rule, and for some time it seemed difficult or even impossible to deal with these apparent anomalies; in fact, none of the proposed divisions into regions can be applied to all cases, even within smaller groups.The correct understanding of this fact, that a large number of animals does not submit to any of the proposed schemes that profess to comply with the present distribution of the condition of life, was made possible by the consideration that the actual distribution of any animal must have originated in the past. Although there are some animals the history of which does not go very far back, in a geological sense, there are others which do, and, generally speaking, we may say that the farther back we go in geological history the more different were the conditions of life from what they are now, and the present distribution of the respective forms must necessarily appear the more strange and anomalous. Wallace, indeed, tried to remove this difficulty in a very peculiar way. He simply propounded his principle of the permanency of the continents, which means to say that the present distribution of land and water (and in general of the physical conditions of life) did not change materially during the earth's history, and that the external features of the earth's surface have remained practically identical from time immemorial up to the present. That this principle is without [[p. 269]] proper foundation has now been recognized and the opposite opinion begins to prevail, that abnormal conditions of distribution are due to just such changes of the physical conditions during a geological past, and that cases of this kind may often enable us to draw conclusions as to the reconstruction of the old conditions. We may safely assume that the character of the physical conditions of the earth's surface has changed continuously and variously in the past and that we possess among living animals many forms which express in their present distribution not only the Tertiary state, but which may also represent Mesozoic or even Palæozoic conditions. Thus it is evident that investigation of the present distribution cannot be used as the starting point for the construction of any scheme. This has been done, however, not only by Wallace--who entirely disregarded the above fact--but also by others, who paid due attention to it. Indeed Osborn2 has pronounced it the purpose of Zoogeography to unite past and present distribution into one scheme, and the same idea has led Jacobi3 to attempt practically this union.
But if we study the most prominent differences between past and present we see that they are chiefly found in the different distribution of land and water, and that frequently in past times land connections existed between parts which are now separated, or vice versa; and thus it is self-evident that the solution of Osborn's problem is simply impossible, since there is no way to express separation and connection of the identical parts in one and the same scheme.4
We consequently arrive at the following three conclusions:
1. Any division of the earth's surface into zoogeographical regions which starts exclusively from the present distribution of animals, without considering its origin, must be unsatisfactory, since always only certain cases can be taken in while others remain outside of this scheme.
2. Considering the geological development of the distribution of [[p. 270]] animals, we must pronounce it impossible to create any scheme whatever that covers all cases.
3. Under these circumstances it is incorrect to regard the creation of a scheme of animal distribution as an important feature or purpose of zoogeographical research.
Thus we are justified in saying that zoogeographical study, as introduced by Wallace, is not directed in the proper channels, and we are confronted with the question, If the creation of regions of animal distribution is not a matter of first importance, which is the vital point in this branch of research?
This question has been practically answered by many writers. I name the following: G. Pfeffer, E. von Ihering, H. A. Pilsbry, R. F. Scharff, C. Hedley, W. Kobelt, H. F. Osborn, A. Jacobi (besides others), and these we may take as representatives of the modern tendency in Zoogeography. According to these authors the chief aim of zoogeographical study consists--as in any other branch of biology--in the demonstration of its geological development. We have to designate this most emphatically, as the final goal of Zoogeography: the retracing of the present animal distribution to its beginning in the past, and a corollary of this is the reconstruction of the ancient physical features of the earth's surface, since these in the first place have guided the development. In the latter respect the distribution of land and water in past times is all-important and the easiest to be traced.
Thus Zoogeography becomes a very important aid not only to physical Geography itself, but also to historic Geology.
The above introductory remarks seem necessary, because the purpose and methods of the new tendency in Zoogeography have been frequently misunderstood, and especially because it was not seen that in this way the fruitless discussions on the limits and value of the different zoogeographical regions, etc., have been rendered unnecessary. Yet it is a habit among zoogeographers to create or discuss zoogeographical regions according to Wallace's ideas, and this is done not only by writers who, like Wallace and Sclater, are principally opposed to any progress in Zoogeography, but also by those who are familiar with the new ideas about the geological development of animal distribution. The old method has become an integral part of this branch of science to such a degree [[p. 271]] that any research in this direction is deemed incomplete that is not finished by the creation or discussion of "regions."In opposition to this, we wish to emphasize that we consider it entirely a matter of indifference whether we accept any regions or not, since none of the possible schemes can be satisfactory. Only in a very limited degree and in a modified sense we believe it advisable to divide the earth into regions, and we have proposed such a division for the marine life districts.5 This scheme, however, is not intended to represent or to express the actual distribution of any animals, but it is a scheme of the distribution of the conditions of existence in the oceans of the present time without consideration of the past or of any definite group of animals. The only purpose of these regions is to single out those marine animals which correspond to the normal conditions of life and to separate them from the abnormal cases; under "normally distributed," consequently, we mean those animals which shape their distribution according to the present features of the earth's surface and which belong in their origin to recent time. All the rest differs and does not fit into these regions; but instead of leaving them out of consideration we know that just these cases are the most interesting, since they demand closer investigation. In most cases we find that these instances of "abnormal" distribution are to be traced back into the geological past in order to be properly understood. This latter study is the most important branch of Zoogeography, and we see that the introduction of "regions" in our method is only the means by which we discover the more interesting and important cases, but it is not the final aim.
Of course the same method may also be used for land and freshwater animals, and it may here be incidentally remarked that the regions proposed by Wallace are in this respect superior to any modifications introduced by later authors, since they generally are well limited and isolated by physical boundaries given on the surface of the earth. But if we are satisfied with the simple statement of the fact that some animals fit into these regions while others do not, we do not approach the solution of the question as to how the actual distribution originated: we are to advance one step further and investigate those cases which do not submit to the scheme. The final aim of this investigation is to compare and group together [[p. 272]] those abnormal cases which resemble each other. Thus we gain certain general views as to ancient geography, and we are finally enabled to trace the distribution of land and water, of climatic conditions and the like in the geological past.
Most prominent among the groups of animals that are available for these investigations are the Mammals, and they have actually been used for just this purpose by various authors (Doederlein, Zittel, Lydekker, Scharff, Osborn). The palæontological material within this group is the most complete of all. But there is one important drawback: since the history of the Mammals hardly goes back beyond Tertiary times, at any rate since the palæontological record of this group is more or less complete only within the Tertiary, we can only draw conclusions from them as to the geographical conditions of this period, while we have to refrain from an investigation of those of the Mesozoic times.
This is a very different matter with the land and freshwater Mollusks. According to what we know, it is apparent that many of these forms can be traced back to Mesozoic times, sometimes even to Palæozoic, and, indeed, it is this group of animals that has furnished the material for the studies of von Ihering, Pilsbry, Hedley, Kobelt, and we are to expect that further investigation in this direction may yield interesting results.
Other groups have also been used. Von Ihering introduced the study of Ants, and there may be other promising groups among the Insects (for instance Spiders). But since the majority of the Insects possess unusual means of dispersal (power of flight) that are apt to obscure the original conditions of distribution, Insects in general are not well adapted to this kind of research. Of other animals the Earthworms have been studied in this respect (by Beddard), and of the Vertebrates, Reptiles, Amphibians, and freshwater Fishes are very likely to prove good objects, since their history in many cases goes back to the beginning of the Mesozoic or even to the Palæozoic time.
In the following treatise I wish to call special attention to certain groups of Decapod Crustaceans that live in fresh water. In part these have been discussed previously by other writers as well as by myself, but it is worth while to go more into detail, since we shall find them very interesting in this respect.
[[p. 273]] The following groups of freshwater Decapods are known:
There are, scattered among other families, other forms of freshwater Decapods, but the above are the most important groups. These are found either exclusively in fresh water or possess the largest number of their members there, and are found only in rare cases in the sea.
As regards the Atyidæ, the present writer has collected the chorological material in a previous paper.7 This is no doubt one of the oldest groups of freshwater Decapods, and their origin, as is very likely also according to their morphological characters, is to be sought for possibly in Jurassic times, although fossil forms are not positively known. The chief features of their distribution are excessively abnormal and even confusing, and therefore the extreme age of the group is again confirmed. On the other hand, there are smaller groups within this family, the distribution of which was apparently formed in later times. Since there is every reason to believe that our knowledge of the actual distribution of the Atyidæ is still more or less defective, we shall refrain from discussing it and refer only to the latest summary given by the present writer.8
In the family of the Palæmonidæ the genus Palæmon forms a group that possesses numerous species which are found chiefly in fresh water. Their distribution, which has also been previously investigated by the present writer,9 points distinctly to the fact that this genus is a very recent one, which is at the present time just in the act of immigrating into fresh water, and that this process is by no means completed. The different species depend in their [[p. 274]] distribution largely on the conditions prevailing in the littoral waters, and generally they follow the physical regions which we have proposed for the marine littoral district of the present time. To this there are only a few exceptions, due to special means of dispersal (crossing over continental divides, for instance). For the investigation of ancient Geography this genus has no value.10
In the following we shall treat of the remaining four families: Potamobiidæ, Parastacidæ, Ægleidæ and Potamonidæ . . .
1This exclusive restriction
to the higher forms of life (Mammals, Birds) is a principle of Wallace
and has been expressly maintained by him as late as in 1894 (see Nature,
Vol. xlix, 1894, p. 610). [[on p. 268]]