CHAPTER VII.

SPECIES AND SPACE.

The geographical distribution of animals presents difficulties.-These not insurmountable in themselves; harmonize with other difficulties.— Fresh-water fishes.-Forms common to Africa and India; to Africa and South America; to China and Australia; to North America and China; to New Zealand and South America; to South America and Tasmania; to South America and Australia.-Pleurodont lizards.-Insectivorous mammals. -Similarity of European and South American frogs. -Analogy between European salmon and fishes of New Zealand, &c.An ancient Antarctic continent probable.—Other modes of accounting for facts of distribution.-Independent origin of closely similar forms. -Conclusion.

THE study of the distribution of animals over the earth's surface presents us with many facts having certain not unimportant bearings on the question of specific origin. Amongst these are instances which, at least at first sight, appear to conflict with the Darwinian theory of "Natural Selection." It is not, however, here contended that such facts do by any means constitute of themselves obstacles which cannot be got over. Indeed, it would be difficult to imagine any obstacles of the kind which could not be surmounted by an indefinite number of terrestrial modifications of surface- submergences and emergencesjunctions and separations of continents in all directions, and combinations of any desired degree of frequency.

All this being supplemented by the intercalation of armies of enemies, multitudes of ancestors of all kinds, and myriads of connecting forms, whose raison d'être may be simply their utility or necessity for the support of the theory of "Natural Selection."

Nevertheless, when brought in merely to supplement and accentuate considerations and arguments derived from other sources, in that case difficulties connected with the geographical distribution of animals are not without significance, and are worthy of mention even though by themselves they constitute but simple problems, the solution or non-solution of which could not alone vitally affect any theory of specific origination.

Many facts as to the present distribution of animal life. over the world are very readily explicable by the hypothesis of slight elevations and depressions of larger and smaller parts of its surface, but there are others which it is much more difficult so to explain.

The distribution either of animals possessing the power of flight, or of inhabitants of the ocean, is of course easily to be accounted for; the difficulty, if there is really any, must mainly be with strictly terrestrial animals of moderate or small powers of locomotion and with inhabitants of fresh water. Mr. Darwin himself observes,1 " In regard to fish, I believe that the same species never occur in the fresh waters of distant continents." Now, the author is enabled, by the labours and through the kindness of Dr. Günther, to show that this belief cannot be maintained; that naturalist having called his attention to the following facts with regard to fish-distribution. These facts show that though only one species which is absolutely 1 "Origin of Species," 5th edition, 1869, p. 463.

and exclusively an inhabitant of fresh water is as yet known to be found in distant continents, yet that in several other instances fishes identical in species (though not exclusively fresh-water) are found in the fresh waters of distant continents, and that very often the same genus is so distributed.

The genus Mastacembelus belongs to a family of freshwater Indian fishes. Eight species of this genus are described by Dr. Günther in his catalogue.1 These forms extend from Java and Borneo on the one hand, to Aleppo on the other. Nevertheless, a new species (M. cryptacanthus) has been described by the same author,2 which is an inhabitant of the Camaroon country of Western Africa. He observes: "The occurrence of Indian forms on the West Coast of Africa, such as Periophthalmus, Psettus, Mastacembelus, is of the highest interest, and an almost new fact in our knowledge of the geographical distribution of fishes."

Ophiocephalus, again, is a truly Indian genus, there being no less than twenty-five species, all from the fresh waters of the East Indies. Yet Dr. Günther informs me that there is a species in the Upper Nile and in West Africa.

The Acanthopterygian family (Labyrinthici) contains nine fresh-water genera, and these are distributed between the East Indies and South and Central Africa.

The Carp fishes (Cyprinoids) are found in India, Africa, and Madagascar, but there are none in South America. Thus existing fresh-water fishes point to an immediate See his Catalogue of Acanthopterygian Fishes in the British Museum, vol. iii. p. 540.

2 Proc. Zool. Soc. 1867, p. 102, and Ann. Mag. of Nat. Hist. vol. xx. p. 110.

3 See Catalogue, vol. iii. p. 469.

connexion between Africa and India, harmonizing with what we learn from Miocene mammalian remains.

On the other hand, the Characinidæ (a family of the physostomous fishes) are found in Africa and South America, and not in India; and even the component groups of the family are so distributed, namely, the Tetragonopterina and the Hydrocyonina.2

Again, we have similar phenomena in that almost exclusively fresh-water group the Siluroids.

3

Thus the Clarias and Heterobranchus 1 are found genera both in Africa and the East Indies. Plotosus is found in

Africa, India, and Australia, and the species P. anguillaris has been brought from both China and Moreton Bay. Here, therefore, we have the same species in two distinct geographical regions. It is however a coast fish, which, though entering rivers, yet lives in the sea.

6

Eutropius is an African genus, but E. obtusirostris comes from India. On the other hand, Amiurus is a North American form; but one species, A. cantonensis, comes from China.

The genus Galaxias has at least one species common to New Zealand and South America, and one common to South America and Tasmania. In this genus we thus have an absolutely and completely fresh-water form of the very same species distributed between different and distinct geographical regions.

Of the lower fishes, a lamprey, Mordacia mordax, is common to South Australia and Chile; while another form

of the same family, namely, Geotria chilensis,1 is found not only in South America and Australia, but in New Zealand also. These fishes, however, probably pass part of their lives in the sea.

We thus certainly have several species which are common to the fresh waters of distant continents, although it cannot be certainly affirmed that they are exclusively and entirely fresh-water fishes throughout all their lives, except in the case of Galaxias.

Existing forms point to a close union between South America and Africa on the one hand, and between South America, Australia, Tasmania, and New Zealand on the other; but these unions were not synchronous any more than the unions indicated between India and Australia, China and Australia, China and North America, and India and Africa.

Pleurodont lizards are such as have the teeth attached by their sides to the inner surface of the jaw, in contra

INNER SIDE OF LOWER JAW OF PLEURODONT LIZARD.

(Showing the teeth attached to the inner surface of its side.)

distinction to acrodont lizards, which have the bases of their teeth anchylosed to the summit of the margin of the jaw. Now pleurodont iguanian lizards abound in the South American region, but nowhere else, and are not as yet known to inhabit any part of the present continent of Africa. Yet pleurodont lizards, strange to say, are found in

1 See Catalogue, vol. viii. p. 509.