Fig. 13.—Distribution of Lemurs.
Madagascar 12 Genera. 36 Species.
Africa, India, Malay 5 Genera. 12 Species.
Madagascar offers a similar case. It abounds with forest vegetation and seems to offer a highly suitable environment for the monkey tribe. Yet there are no apes on the island. Their place is occupied by the Lemur tribe, which, there is every reason to believe, is the older group of the two, and that from which the apes have sprung. It is supposed, then, that Madagascar was separated from Africa before the ape had evolved. The lemurs thenceforward were free from the competition of their more highly developed relatives, and have branched out into a great variety of types, while still remaining on a relatively low plane of intelligence and specialisation. The distribution of the Lemurs is shown in Fig. 13.
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.
Fig. 14.—Stages in development of embryo of rabbit.
a, 10 days; b, 11 days; c, 15 days; d, 17 days old.
