brata there were fishes not belonging to the lowest but to the very highest groups which are known to have ever been developed, namely, the Elasmobranchs (the highly-organized sharks and rays), and the Ganoids, a group now poorly represented, but for which the sturgeon may stand as a type, and which in many important respects more nearly resemble higher Vertebrata than do the ordinary or

osseous fishes. Fishes in which the ventral fins are placed in front of the pectoral ones (i.e., jugular fishes) have been generally considered to be comparatively modern forms. But Prof. Huxley has kindly informed me that he has discovered a jugular fish in the Permian deposits.

Among the molluscous animals we have members of the very highest known class, namely, the Cephalopods, or cuttle-fish class; and among articulated animals we find Trilobites and Eurypterida, which do not belong to any

incipient worm-like group, but are distinctly differentiated. Crustacea of no low form.

We have in all these animal types nervous systems differentiated on distinctly different patterns, fully-formed organs of circulation, digestion, excretion, and generation, complexly-constructed eyes and other sense organs; in fact, all the most elaborate and complete animal structures built up, and not only once, for in the fishes and mollusca we have (as described in the third chapter of this work) the coincidence of the independently-developed organs of sense attaining a nearly similar complexity in two quite distinct forms. If, then, so small an advance has been made in fishes, mollusks, and anthropods, since the Upper Silurian deposits, it will probably be within the mark to consider that the period before those deposits (during which all these organs would, on the Darwinian theory, have slowly built up their different perfections and complexities) occupied time at least a hundredfold greater.

Now it will be a moderate computation to allow 25,000,000 years for the deposition of the strata down to and including the Upper Silurian. If, then, the evolutionary work done during this deposition only represents a hundredth part of the sum total, we shall require 2,500,000,000 (two thousand five hundred million) years for the complete development of the whole animal kingdom to its present state. Even one-quarter of this, however, would far exceed the time which physics and astronomy seem able to allow for the completion of the process.

Finally, a difficulty exists as to the reason of the absence of rich fossiliferous deposits in the oldest strataif life was then as abundant and varied as, on the Darwinian theory, it must have been. Mr. Darwin himself admits 17 the case at present must remain inexplicable; and may be truly urged as a valid argument against the views" entertained in his book.

66

17 "Origin of Species," 5th edit., p. 381.

Thus, then, we find a wonderful (and, on Darwinian principles, an all but inexplicable) absence of minutely transitional forms. All the most marked groups, bats, pterodactyls, chelonians, ichthyosauria, anoura, etc., appear at once upon the scene. Even the horse, the animal whose pedigree has been probably best preserved, affords no conclusive evidence of specific origin by infinitesimal, fortuitous variations; while some forms, as the labyrinthodonts and trilobites, which seemed to exhibit gradual change, are shown by further investigation to do nothing of the sort. As regards the time required for evolution. (whether estimated by the probably minimum period required for organic change, or for the deposition of strata which accompanied that change), reasons have been suggested why it is likely that the past history of the earth does not supply us with enough: First, because of the prodigious increase in the importance and number of differences and modifications which we meet with as we traverse successively greater and more primary zoological groups; and, secondly, because of the vast series of strata necessarily deposited if the period since the Lower Silurian marks but a small fraction of the period of organic evolution. Finally, the absence or rarity of fossils in the oldest rocks is a point at present inexplicable, and not to be forgotten or neglected.

Now all these difficulties are avoided if we admit that new forms of animal life of all degrees of complexity appear from time to time with comparative suddenness, being evolved according to laws in part depending on surrounding conditions, in part internal-similar to the way in which crystals (and, perhaps from recent researches, the lowest forms of life) build themselves up according to the internal laws of their component substance, and in harmony and correspondence with all environing influences and conditions.

CHAPTER VII.

SPECIES AND SPACE.

The Geographical Distribution of Animals presents Difficulties.-These not insurmountable in themselves; harmonize with other Difficulties.-Frosh-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, etc.-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. Among 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 by 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 emergences-junctions 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 feeble and more or less easily explicable puzzles which could not alone suffice either.to sustain or to defeat any theory of specific organization.

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 the existence of which it is much more difficult so to explain.

1

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 66 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 labors and through the kindness of Dr. Günther, to show that this belief cannot be maintained; he having been so obliging as to call attention to the following facts with regard to fish-distribution. These facts show that though only one species which is absolutely and exclusively an inhabitant of fresh water is as yet known to be found in distant continents, yet that in several other instances the same species is found in the fresh water 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 de

1 "Origin of Species," 5th edit. 1869, p. 463.