In Dr. Alfred Russel Wallace's book on _Island Life_ there are set forth a great number of interesting facts on the subject of the animal population of islands, and many striking interpretations of these facts in the light of the Evolution theory. Coral islands, and those caused by volcanic eruptions, are peopled with inhabitants which have accidentally come thither by flight, or have been brought, for example, on floating timber by ocean currents. On the other hand, islands which represent separated fragments of continents have usually a fauna of the same general type as that of the continent of which they have formed a part. But the actual species are frequently different, and if the separation is of more ancient date, the differences are still more marked. The fact of this divergence of an isolated animal population from that from which it has originated is sufficiently striking, and would remain an inexplicable problem, were we without an Evolution theory. According to the Evolution hypothesis, however, the restricted and somewhat special environment favours a modification of the original types with which the island was provided, and a satisfactory explanation is offered. Finally, we may mention the evidence that has been gathered from the study of embryology and development. It has been stated, in a metaphor which is perhaps more clever than it is exact, that every animal climbs up its own ancestral tree; and while it would be absurd to say, for instance, that a mammalian embryo resembles successively a fish, an amphibian, and a reptile, still many of the broad facts in the evolution of a race seem to be repeated, in a more or less blurred and indistinct fashion, in the development of the individual. Thus, for example, gill-slits and a tail are possessed in common by the embryos of all higher animals, only afterwards to disappear in those types in which the adult animal is without these structures. The heart of the mammal or bird is at first simple, then two chambered like that of a fish, then three chambered like an amphibian's, and finally four chambered. Some of the main phases in the development of the rabbit and of man are shown in Figs. 14 and 15 respectively. [Illustration: FIG. 14.--Stages in development of embryo of rabbit. _a_, 10 days; _b_, 11 days; _c_, 15 days; _d_, 17 days old.] [Illustration: FIG. 15.--Stages in development of human embryo. _a_, 18-21 days; _b_, 27-30 days; _c_, 35 days; _d_, 52-54 days old.] The young flat-fish is like an ordinary member of the fish tribe, with an eye on either side of its head, and its body built on the ordinary symmetrical lines. It is only later, when it begins habitually to be upon one side on the sea bottom, that the eye from the under side wanders round to the opposite aspect beside its fellow, and the upper side becomes pigmented, while the lower remains white.