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ANIMALS AND THEIR NATIVE
IN the December number of this Review, Mr. P. L. Sclater called attention to the subject of the geographical distribution of animals in its bearing on the theory of evolution, and gave numerous special cases in which the actual distribution of particular species and groups is very difficult to explain on that theory without making assumptions which, in his opinion, the evidence at our disposal does not warrant. Difficulties of this nature are so numerous, and many of them seem to him so weighty, that, in order to explain them, he is led to question, what is almost an axiom with evolutionists, that identity of structure is, without exception, an indication of descent from a common parent. Similar doubts, though not stated in exactly the same terms, have been felt by Professor Mivart; and it therefore becomes a matter of interest to examine a little more closely into the alleged difficulties, in order to see whether they are not really explicable on the principle of descent with modification, only calling to our aid such general assumptions as are fully warranted by what we actually know of the migrations and extinctions of living things, and of the past changes in the physical condition of the earth and its inhabitants.
As Mr. Sclater's article gives an excellent summary of the nature and meaning of zoological distribution, and of the main general conclusions arrived at by naturalists, our purpose will be best attained by proceeding at once to consider his special cases of difficulty; and in doing so we shall have occasion to discuss, as fully as may be required, the general principles and particular illustrations needed to elucidate them.
We have first the case of the Little Blue Magpie of Spain, which has a very close ally in the extreme parts of Eastern Asia and Japan, while there is nothing closely allied to these in all the intervening regions or in any other part of the world. This is said to be an infringement of the canon as to the continuity of specific areas, and as such to require explanation. Before proceeding further, it will be well to inquire into the value of this canon of continuity,
1 Genesis of Species, chap. iii.
and whether it is so clear and well established that infringements of it are altogether exceptions to the usual course of nature. So far from this being the case, I believe it will be found that, between the complete continuity of the area occupied by a species or a genus and such wide discontinuity as occurs in the present example, there is every possible gradation; and further, that the instances of discontinuity are very numerous, while those of complete continuity are far less generally the rule than appears at first sight.
In order to understand the bearing of this class of phenomena on the theory of derivation, let me briefly indicate the probable course of a genus of animals from its birth or origin to its final extinction.
Genera are groups of species which agree among themselves, and differ from all other groups in the same family or order, by the possession of some structural peculiarities. We must therefore suppose a genus to have had its origin in some variation of structure which was useful to its possessors-such as a modification of the bill, feet, or wings of a bird, or of the teeth, claws, or horns of a mammal. According to the theory of natural selection, the possessors of such a useful peculiarity would increase at the expense of their close allies who did not possess it, and would soon form a distinct group of individuals breeding together and constituting a species-the first species of the new genus. This species having in time supplanted the parent species, and being better adapted than it for the battle of life, would almost certainly cover a wider area, and thus come into competition with several of the allied species of the old genus, some of which it would also probably supplant, and occupy the areas they formerly occupied. But as they had been modified into distinct species (differing, perhaps, slightly in colour or habits in accordance with the varying physical conditions), so the now widespread species of the new genus would vary, and become modified in a somewhat analogous manner, forming a genus consisting of several species. Now, if the generic form thus produced was one of great inherent vigour and adaptability, and if the peculiarity of structure it possessed was of considerable importance, it would become what Mr. Darwin terms a dominant group: that is, it would spread widely over the earth under various modified forms suited to the various conditions it became subject to. At last it would reach its maximum of development, and cease to spread further, either owing to its inability to adapt itself to further changes of climate, &c., or, what is more likely, from its coming into competition with other dominant groups which had in like manner spread from some other centres.
Now, during all this time, which may be termed the period of growth of the genus, its area will have been almost necessarily continuous, and the areas occupied by its several species (also continuous) will probably overlap each other. But now commences its period of decay. Other groups of the same or allied families have given rise
to varieties which have also become dominant species and genera, which, under the somewhat changed physical conditions that in time have come about, beat it in the battle of life, and force it to retire step by step from the vast area it had overrun. First one species and then another will dwindle away and become finally extinct, and by so doing will necessarily leave gaps in its area of distribution. This process going steadily on, the time will at last come when two or three species only will remain, most likely in widely separated parts of its former area; their position being determined either by the competition being there somewhat less severe, or by some speciality of conditions which are exceptionally favourable to the dying-out group. Then one and then another of these species will die out, and the once extensive genus will only be represented by a single species inhabiting a very restricted locality. This will become rarer and rarer, the necessary preliminary to that final extinction which we know to be the fate, sooner or later, of every group of living things.
Most working naturalists (and none better than Mr. Sclater) are acquainted with genera whose distribution will illustrate all the successive phases of this hypothetical history; while palæontology furnishes us with some actual examples of the progress of a group from its rise to its decay, though, owing to the extreme imperfection of the geological record (and its total absence for important epochs in many parts of the globe), we can never trace the complete history of such a group. A little consideration will show us, however, why it is that continuity of generic and specific areas appears to be the rule, discontinuity the exception. There can be no doubt that the development of an extensive genus is a slow process, while its decay and final extinction need not be slow, and may conceivably be extremely rapid. Geological and geographical changes may be long in preparation, but finally very abrupt. Land may sink a thousand feet without producing any very important effect except diminution of area, but the next hundred feet of depression may cut it off from a continent, and may alter the direction of ocean currents, thus producing a greater organic and physical change than had been brought about by the previous subsidence occupying ten times as long. Again, such a change as that which admitted the highly organised Miocene mammalia of Europe into Tropical and South Africa must have led at once to the extermination of many of the indigenous species, and have restricted the area of many more. It is also important to remember that the dominant or growing species and genera, which are those having continuous areas, will be necessarily more prominent, more numerous in species and individuals, and therefore far better known; while those in process of extinction, and for that very reason having discontinuous areas, will be less numerous, far less common, and in fact often very rare, and therefore much less known. In many cases, too, it will happen that the discontinuity
is not great as regards distance, and it will then not be noticed, or will be imputed to want of knowledge, although it may be quite as real as when half a continent lies between the two species.
It appears, therefore, that the discontinuity of many genera and higher groups, so far from being difficult of explanation, is really one of the inevitable results of the process of extinction which is always going on. The peculiarity of the particular case we are considering is that it is somewhat extreme in the fact of two species only being left, occupying limited areas situated at the opposite extremities of the immense Palearctic region. But this is not very extraordinary, because there are in Western Europe and Japan a number of pairs of closely allied species whose extinction in the intervening areas would lead to an exactly similar phenomenon to that we are considering. Such are the European and Japanese jays, bullfinches, goldcrest warblers, and wrens, all of which are closely allied to each other, while they are separated by a wide area in Central Asia often occupied by species which differ considerably from both. Should either of these groups die out, we might expect that the species inhabiting the comparatively desert and inhospitable regions of Central Asia would succumb first, while those living in the milder and more equable climates of Western Europe and Japan would probably linger on, the last of their race. It is very interesting to note that in most cases of such widely separated but closely allied species or groups there is a decided similarity in the general physical conditions of the countries they inhabit. The ally of the Spanish blue magpie is found in North-eastern Asia from Shanghai to Pekin and the Amoor, as well as in Japan; and these countries reproduce the hot summers and the cold winters, the rugged mountains and the sheltered valleys of Spain, while both areas are subject to the influence of the vicinity of the ocean in an almost equal degree.
Before going further we must guard against a misconception as to the progressive rise and decay of species, genera, and higher groups. It is not maintained that this will always take place uninterruptedly or continuously. On the contrary, it is certain that the decay of a group may run its course for a time, and then, owing to changed conditions, may be checked, and even be changed into a new growth and development. Hence arise those isolated groups, which yet, by their abundance in species and the considerable area they occupy, show that they are in a flourishing condition-of which the tanagers in South America, the broadbills (Eurylamida) of Asia, and the colies of Africa, may serve as examples. When an extensive group is in process of extinction, it may become broken up into many isolated portions, sometimes in juxtaposition to each other, sometimes separated in remote parts of the globe. Changes of conditions, whether physical or organic, may favour first one, then
another, of these portions, or the same portion may be subject to alternate phases of progression and decay several times repeated. How can we wonder that the final result of such complex processes, whose general nature we can understand, but whose details it is impossible for us to trace, should often lead to anomalies in geographical distribution? And when we consider that these processes have been often intensified and further complicated by geographical mutations, and by those forced migrations induced by the climatal changes which culminated in the glacial epoch, the wonder rather is that we can account for so much, than that there should be matters of detail which we cannot explain.
It is clear, then, that the case of the Little Blue Magpie of Spain and its close ally in Eastern Asia is simply an example of a dyingout group, of which two species only remain isolated in countries favourable to their existence; and further, that the supposition of their common ancestors having once occupied the intervening region, so far from being unwarranted, is supported by the analogy of several other groups of birds in the same area.
Mr. Sclater's next two cases may be sufficiently explained by the application of the general considerations and examples already adduced. We have two allied species of Oxyrhamphus isolated in South-East Brazil and Central America-countries, it may be remarked, about equally removed from the equator and enjoying very similar climates; while in the cuckoos of the genus Neomorpha we have a similar phenomenon in nearly the same two areas, with the addition of three species in the intervening districts which are not closely allied to the two others.
We have here really only the same class of facts as occur plentifully in the Palearctic region, in which the species of the eastern and western extremes are often alike, while those that intervene are
more diverse. We can, without much difficulty, refer this latter peculiarity to diversity or similarity of climate and physical conditions, while in the Neotropical region it is more probable that a diversity of organic conditions may have been the agent at work. There is some reason to believe that the great plateau of Guiana long formed an island, and that this isolation led to the development of several peculiar forms, which have in some cases spread into Upper Amazonia. A range of plateaux and hills, on the other hand, connects Brazil with the Andes, and has thus kept up a greater zoological continuity with Central America than the intervening area of Guiana has been able to do.
Mr. Sclater's third problem, that of Pitta angolensis, is very interesting, and will afford us an opportunity of discussing some of the most curious phenomena of distribution, and of bringing forward some considerations which I believe will go far towards the removal of most of the difficulties they present. The case is that of an ex