RECENT GEOGRAPHICAL AND HISTORICAL

PROGRESS IN ZOOLOGY.

IN

N few departments of natural science have so many definite and well-marked advances been attained, within the last few years, as in geographical and historical investigation. The principles of general classification have been gradually but continuously developed by successive investigators. Comparative anatomy has been built up by the cumulative labours of one generation after another, until its student now stands on the elevated platform where the genius of Owen has placed him; but in its geographical and historical aspects, the study of biology has only within our own times obtained its due. consideration. Histology, indeed, the examination of the ultimate structure of organized bodies in relation to their development,—was wholly unknown until the observations of the microscope were brought to bear on zoology: but it is beyond our present purpose to offer any remarks on the progress of animal physiology.

If we attempt to trace the history of biological research we shall find that we have, in Aristotle and Pliny, "a period of unsystematic knowledge;" then, from the revival of letters, to Willoughby and Ray, a period of misapplied erudition," exemplified in Aldrovandus; epoch of the discovery of fixed characters (Linnæus); a period in which many systems were put forward; a struggle of an artificial and a natural method, and a gradual tendency of the natural

an

method to a manifestly physiological character."* From the time of Aristotle, in whose writings there are, amidst a stupendous collection of facts, frequent glimpses of his view of the necessity of some zoological system, though scarcely so clearly developed as some of his admirers would lead us to admit, we have to wait for many dreary centuries before we meet with any worthy successor. Till modern times not a single additional step was made. With Willoughby and Ray, in our own country, natural history began to be again studied in nature, and the business of classification necessarily, in the first instance, forced itself upon the attention, to the exclusion of all other considerations, until Linnæus perfected his "Systema Naturæ." During this period the general physiological features of animals were directly and evidently the primary subjects of study, and attracted students to comparative anatomy, almost to the exclusion of what may be termed the minor branches of zoological science. Had it not been for such patient and long-continued toil, and for the light thrown on systematic zoology by physiology, geographical and historical zoology would have been impossible, or even more speculative than the many schemes of artificial classification which have been put forth. For whatever partial truth may underlie the doctrine of a circular progression in the series of affinity, and of a quinary division of such circular groups, or of the relation of analogy between the members of such groups as distinct from the relation of affinity, it must be admitted that our accumulation of facts is not yet sufficiently exhaustive to afford complete demonstration of any such theories.

For the most part, since the time of Linnæus, naturalists had been employed rather in the collection of a vast accumulation of facts and specimens, heaping up the materials of the language, compiling a vocabulary of zoology, the subject-matter for future systematists, than in forming and arranging those materials into the shape of a grammar and an accidence. Not that their labours were useless or superfluous, they did but ascertain and record the facts which are now beginning to enable us to understand the language of Nature, and to read her lessons in the vernacular.

It was impossible for any student of nature, after the epoch of Linnæus, to overlook the fact that certain types of animal life, as well as particular species, were characteristic of particular regions of the earth. When, for example, the fauna and flora of Australia first attracted attention, so strange and anomalous were the types of that continent, that there was a difficulty in finding any place for many of them in the existing systems of naturalists. It had long been known that marsupial mammals were the predominant type of their class in Australia (the single exception, beyond the rodents and bats, being

Whewell, "Inductive Sciences," iii. 297.

the dingo dog, so doubtfully indigenous as scarcely to deserve mention as exceptional), that wingless birds were characteristic of New Zealand, and that humming birds were peculiar to the New World. But alongside of these general limitations, naturalists were led to estimate, as of co-ordinate value in geographical classification, the narrow limits within which a very large proportion of the species of vertebrata are circumscribed, especially the larger and more remarkable mammals, as the orang-outang upon the Sunda Isles, the chimpanzee and gorilla on the West Coast of Africa, the common tapir in South America, the Eastern tapir in Sumatra only, the Bactrian camel and the dromedary in Asia, the llama in a limited district of the Andes. And in like manner, in the lower groups, the Proteus anguinus in the caves of Carinthia, or the blind fish of the mammoth cave of Kentucky. In some respects every island of the Pacific, as well as each group of the isles which stud the North Atlantic, and in which distinct animals are found, may be considered as exhibiting a distinct fauna. Yet all the groups of the Atlantic-the Canaries, Madeiras, Cape Verde, Azores, and St. Helena-have a common character, which unites them in a more comprehensive fauna. Still more is this true of the Pacific Islands, where the points of resemblance are not confined to molluscs and insects, the only classes left from which to form an induction as to the geographical relations of the old Atlantis, but where a rich bird fauna still exists, marvellously varied, from the Sandwich Islands to the Feejees and New Zealand, yet having sufficient generic affinities to enable us to recognise a similarity of type, pointing probably to a common origin, and indicating unmistakeably a relationship, either geographical or historical, more or less remote. It has been remarked with truth that

"We must distinguish between zoological realms, zoological provinces, zoological countries, zoological fields, as it were, that is, between zoological areas of unequal value, over the widest of which range the most extensive types, while in their smaller divisions we find more and more limited types, sometimes overlapping one another, sometimes placed side by side, sometimes concentric to one another, but always and everywhere impressing a special character upon some part of a wider area, which is thus made to differ from any other part within its natural limits."*

Yet so little were these principles recognised by the earlier observers, that we find them enumerating without suspicion, in their lists of the fauna of one region, species which they identified with those of a distant part of the earth under different conditions, and which did not occur in the intervening regions. Thus Russell, in his "Natural History of Aleppo," more than a century ago, had no difficulty in identifying Syrian birds with American species then recently described by Linnæus.

* Agassiz on Classification, p. 49.

*

It is only eight or nine years since Mr. Sclater brought before the Linnean Society those views on geographical distribution which have been at once adopted and acknowledged by the whole scientific world, and further investigation has proved that the "regions" sketched out by the learned writer of the "Geographical Distribution of Birds" are equally applicable to every other department of zoology, as well as to botany.

Mr. Sclater divides the whole living creation into six distinct regions, the Palearctic, or Northern Old World; the Nearctic, or North American; the Ethiopian; the Indian; the Australian, and the Neotropical, or South American regions. The Palearctic region comprises all Europe, Africa north of the Sahara, and all Continental Asia north of about the parallel 30° N. lat., with Japan and the Kurile Islands. The Nearctic includes Greenland and all North America, down to about 22° N. lat.; the line reaching farther north on the coast, and running farther south in the central mountainous portion. The Ethiopian region comprehends all Africa south of the Sahara, with Madagascar, Mauritius, Bourbon, &c., and all Western and Southern Arabia. The Sahara, destitute of indigenous animal life, must be excluded from either region, and looked upon as an area of sea. The Indian region embraces Eastern Arabia and all Continental Asia, south of the Himalayahs, and of about 30° N. lat.; together with a very large and definite portion of the Eastern Archipelago, marked by a line drawn between the islands of Bali and Lombok, between Borneo and Celebes, and between the Philippines and the Moluccas. The Australian region extends over all the islands south of the above-named line, as well as the Continent of Australia, New Zealand, and all the island groups of the Pacific Ocean. Lastly, the Neotropical stands for South America, the West Indies, and that portion of the Spanish Main lying south of 22° N. lat.

By these divisions it is not meant that no character of one region impinges on the boundaries of the adjacent one, still less that identical species are conterminous universally, or even generally, with its whole area, but that in all the products of each, animal or vegetable, there is for the most part a distinct generic stamp, and that when the conditions of existence are identical in different regions, the species of the one are represented either by forms of life specifically distinct, or more frequently by widely varied genera, performing like functions, and filling a similar place in the economy of nature. Some specific forms may have a range co-extensive with the whole region, as witness the many species of birds common to the British Isles and to Japan, not one of which is found in North America. Others are restricted to the narrowest limits, as the Nestor productus of Philip Island, the

* Journ. Lin. Soc. Zool., ii. 130.

dodo of Mauritius, the solitaire of Bourbon, or those species of humming-bird whose range appears to be bounded by the crater of a single extinct volcano. Still all these are moulded on the type of the region to which they pertain, or, if you will, are developments of its type. Again, many species of one region have overspread a large portion of an adjacent one, where the conditions of life were favourable to their increase: thus the wild boar of the Palearctic region has peopled Continental India. But when we pass to the islands which it could not reach, owing to its geologically recent introduction into the Peninsula, there we find other species, as the babirussa, occupying a parallel position in the natural order of things. So in the Ethiopian region various other distinct species of the swine group occur, while the peccary is its representative in South America.

When we turn to the invertebrate animals, we find that this grand division into six regions holds equally good: thus while the lepidoptera of the Palearctic and Nearctic regions are almost invariably specifically distinct, yet there is the same parallelism of type, the climatic and other conditions being similar. In like manner there is a parallelism, without any specific identity, between the butterflies of South America and of the Indian regions: the gorgeous papiliones, for example, abound in both, but are never specifically identical. Again, while Mr. Sclater's division was founded exclusively on a consideration of the higher vertebrates, it has received no more conclusive corroboration than from the grouping of land shells, perhaps the least locomotive of all living forms, and of which the species are for the most part limited to the most circumscribed areas. In this class there are several great divisions, like the Linnean genera of Helix, Bulimus, and Achatina, which are found universally throughout the world, comprising several thousands of species. Yet while the conditions of Europe and of a great part of North America are almost identical, as are those of India and South America, there is scarcely a species common to any two zoological regions. Nor is this all. While the species from extreme points of the same region exhibit much variety, there is a homogeneity of type which would render it impossible to separate at sight the land shells of Northern China, for instance, from those of France. But through all the species of North America there is a certain character by which every naturalist could at once, with tolerable accuracy, pronounce, though he had never seen the shell before, that it belonged not to the Palearctic but to the Nearctic fauna. The same remark holds true of the Australian, Neotropical, and Ethiopian land molluscs. We believe that we shall be supported by every student of classification in asserting that this division of regions will bear the test of each order of the lower animals, as it undoubtedly does of botany.