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Extinction of Species: Darwin

The Tsetse fly extirpates all horses, dogs, and cattle, from certain districts of South Africa, and a representative species in Paraguay is equally fatal to new-born cattle and horses. Mr. Darwin * shows that the struggle is more severe between species of the same genus, when they come into competition with each other, than between species of distinct genera. Thus one species of swallow has recently expelled another from part of the United States; and the misselthrush has driven the song-thrush from part of Scotland. In Australia the imported hive-bee is rapidly exterminating the small stingless native bee, and similar cases might be found in any number.

Mr. Wallace, in quoting Mr. Darwin as to these facts, points the conclusion that "any slight change, therefore, * Origin of Species, C. Darwin. 5th edit. 1869. of physical geography or of climate, which allows allied species hitherto inhabiting distinct areas to come into contact, will often lead to the extermination of one of them." It is the province of the palæontologist to enumerate the many remarkable forms which have passed away since man's first appearance upon the globe, and to trace their fluctuations over both hemispheres as determined by the advance and retreat of glacial conditions, and by the protean forms assumed by past and existing continents under oscillations of elevation and depression. Many interesting points, such as the dates of the successive separation of Ireland and Great Britain from the main continent, can be determined with accuracy from the record furnished by the fossil remains of animals of those times; and many interesting associations of animals with man at various dates, in our present island home and in other countries, have been traced by the discovery of their remains in connection with his, in bone deposits in caverns and elsewhere.

Conversely, most valuable deductions are drawn by the zoologist from the review which he is enabled to take, through the connected labours of his colleagues in all departments, of the distinct life regions now mapped out upon the face of the globe. These, after the application of the necessary corrections for various disturbing or controlling influences referred to above, afford proof reaching far back into past periods, of successive alterations in the disposition of continents and oceans, and of connections long since obliterated between distant lands.

The palæontologist reasons from the past to the present, the zoologist from the present to the past; and their mutual labours explain the evolution of existing forms, and the causes of the disparity or connection between those at present characterizing the different portions of the surface of the globe.

The palæontologist, for example, traces the descent of the horse, which, until its reintroduction by the Spaniards was unknown in the New World, through a variety of intermediate forms, to the genus Orohippus occurring in Eocene deposits in Utah and Wyoming. This animal was no larger than a fox, and possessed four separated toes in front, and three behind. Domestic cattle he refers to the Bos primigenius, and many existing Carnivora to Tertiary forms such as the cave-bear, cave-lion, sabre-tiger, and the like. The zoologist groups the existing fauna into distinct provinces, and demands, in explanation of the anomalies which these exhibit, the reconstruction of large areas, of which only small outlying districts remain at the present date, in many instances widely separated by oceans, though once forming parts of the same continent; and so, for the simile readily suggests itself, the workers in another branch of science, Philology, argue from words and roots scattered like fossils through the various dialects of very distant countries, a mutual descent from a common Aryan language: the language of a race of which no historical record exists, though in regard to its habits, customs, and distribution much may be affirmed from the large collection of word specimens stored in philological museums. Thus Mr. Sclater, on zoological grounds, claims the late existence of a continent which he calls Lemuria, extending from Madagascar to Ceylon and Sumatra; and for similar reasons Mr. Wallace extends the Australia of Tertiary periods to New Guinea and the Solomon Islands, and perhaps to Fiji, and from its marsupial types infers a connection with the northern continent during the Secondary period. Again, the connection of Europe with North Africa during a late geological period is inferred by many zoologists from the number of identical species of mammalia inhabiting the opposite sides of the Mediterranean, and palæontologists confirm this by the discovery of the remains of elephants in cave-deposits in Malta, and of hippopotami in Gibraltar; while hydrographers furnish the supplemental suggestive evidence that an elevation of only fifteen hundred feet would be sufficient to establish two broad connections between the two continents so as to unite Italy with Tripoli and Spain with Morocco, and to convert the Mediterranean Sea into two great lakes, which appears, in fact, to have been its condition during the Pliocene and Post Pliocene periods. It was by means of these causeways that the large pachyderms entered Britain, then united to the continent; and it was over them they retreated when driven back by glacial conditions, their migration northward being effectually prevented by the destruction of the connecting arms of land. Some difference of opinion exists among naturalists as to the extent to which zoological regions should be subdivided, and as to their respective limitations. But Mr. A. R. Wallace, who has most recently written on the subject, is of opinion that the original division proposed by Mr. Sclater in 1857 is the most tenable, and he therefore adopts it in the very exhaustive work upon the geographical distribution of animals which he has recently issued. Mr. Sclater's Six Regions are as follows :- 1.-The Palearctic Region, including Europe, Temperate Asia, and North Africa to the Atlas mountains. 2.-The Ethiopian Region, Africa south of the Atlas, Madagascar, and the Mascarene islands, with Southern Arabia. 3. The Indian Region, including India south of the Himalayas, to South China, and to Borneo and Java. 4. The Australian Region, including Celebes and Lombok, Eastward to Australia and the Pacific islands. 5.-The Nearctic Region, including Greenland, and North America, to Northern Mexico. 6.-The Neotropical Region, including South America, the Antilles, and Southern Mexico.

This arrangement is based upon a detailed examination of the chief genera and families of birds, and also very nearly represents the distribution of mammals and of reptiles. Its regions are not, as in other subsequently proposed and more artificial systems, controlled by climate; for they range, in some instances, from the pole to the tropics. It probably approaches more nearly than any other yet proposed to that desideratum, a division of the earth into regions, founded on a collation of the groups of forms indigenous to or typical of them, and upon a selection of those peculiar to them; with a disregard of, or only admitting with caution, any which, though common to and apparently establishing connection between two or more regions, may have in fact but little value for the purpose of such comparison; from the fact of its being possible to account for their extended range by their capability of easy transport from one region to another by common natural agencies.* Such an arrangement should be consistent with the retrospective information afforded by paleontology; and, taking an extended view of the subject, be not merely a catalogue * Thus Mr. Wallace considers that the identity of the small fish, Galaxias attenuatus, which occurs in the mountain streams of Tasmania, with one found in those of New Zealand, the Falkland Islands, and the temperate regions of South America, cannot be considered as demonstrating a land connection between these places within the period of its specific existence. For there is a possibility that its ova have been transported from one point to another on floating ice; and for similar reasons fresh-water fish generally are unsafe guides to a classification of zoological regions. Mr. Darwin has shown (Origin of Species, and Nature, vol. xviii. p. 120 and vol. xxv. p. 529) that mollusca can be conveyed attached to or entangled in the claws of migratory birds. Birds themselves are liable to be blown great distances by gales of wind. Beetles and other flying insects may be similarly transferred. Reptiles are occasionally conveyed on floating logs and uprooted trees. Mammals alone appear to be really trustworthy guides towards such a classification, from their being less liable than the other classes to accidental dispersion. of the present, but also an index of the past. It should afford an illustration of an existing phase of the distribution of animal life, considered as the last of a long series of similar phases which have successively resulted from changes in the disposition of land and water, and from other controlling agencies, throughout all time. A reconstruction of the areas respectively occupied by the sea and the land at different geological periods will be possible, or at least greatly facilitated, when a complete system of similar groupings, illustrative of each successive period, has been compiled. It is obvious that any great cosmical change, affecting to a wide extent any of the regions, might determine a destruction of specific existence; and this on a large scale, in comparison with the change which is always progressing in a smaller degree in the different and isolated divisions. The brief remarks which I have made on this subject are intended to suggest, rather than to demonstrate--which could only be done by a lengthy series of examples-the causes influencing specific existence and its in many cases extreme frailty of tenure. And I shall now conclude by citing from the works of Lyell and Wallace a short list of notable species, now extinct, whose remains have been collected from late Tertiary, and Post Tertiary deposits that is to say, at a time subsequent to the appearance of man. From other authors I have extracted an enumeration of species which have become locally or entirely extinct within the historic period.

These instances will, I think, be sufficient to show that, as similar destructive causes must have been in action during pre-historic times, it is probable that, besides those remarkable animals of which remains have been discovered, many others which then existed may have perished without leaving any trace of their existence. There is, consequently, a possibility that some at least of the so-called myths respecting extraordinary creatures, hitherto considered fabulous, may merely be distorted accounts-traditions of species as yet unrecognised by Science, which have actually existed, and that not remotely, as man's congener.