34 GENERAL PRINCIPLES OF ZOOLOGY 



tion with the other continents. While in the other parts of the earth the 

 higher vertebrates, which were developed from the marsupials and their 

 lower contemporaries, came, by way of the lands connecting the various 

 continents, to have a wide distribution, in isolated Australia this process 

 of evolution did not go on, and its ancient faunal character was preserved. 

 (2) As Wallace has shown, the Malay Archipelago is divided faunally 

 into an eastern and a western half. The fauna of the first has a thoroughly 

 Australian character; that of the latter recalls Farther India and the 

 Oriental Province. Differences in climate and vegetation cannot be the 

 cause of this, since in both there are islands with dry and others with 

 moist climates, with sparse and with luxuriant vegetation. The only ex- 

 planation is that the eastern Malay Islands have developed geologically 

 in connection with Australia, the western with India. Wallace tried 

 to draw a sharp line ('Wallace's line') between the two regions, passing 

 between the islands of Bali and Lombok. Later studies have not confirmed 

 this, but have shown that between the two regions is a zone of islands 

 (including Celebes) in which a mixture of faunas occurs. (3) Long 

 before Darwin, the geologist von Buch, from the distribution of plants on 

 the Canary Islands, came to the conclusion of a change of species into new 

 species; viz., on islands peculiar species develop in secluded valleys, be- 

 cause high mountain-chains isolate plants more effectually than do wide 

 areas of water. Moritz Wagner has collected many instances which prove 

 that localities inhabited by certain species of beetles and snails have been 

 sharply divided by wide rivers or by mountain-chains, while in neighbor- 

 ing regions related so-called 'vicarious species' are found. The peculiar 

 character of the fauna and flora of isolated island groups also needs 

 mention. The Hawaiian Islands have no less than 70 endemic birds out 

 of a total of 116, the Galapagos 84 out of 108. 



Causal Foundation of the Theory of Evolution. The Darwinian 

 theory, so far as the above exposition shows, is fundamentally like the 

 theories of descent advocated at the beginning of the last century by 

 Lamarck and other zoologists; it is distinguished from these only by its 

 much more extensive foundation of facts, and further in that it abandoned 

 the successional arrangement and replaced it by the branched, tree-like 

 mode of arrangement the genealogical tree. But still more important 

 are those advances which relate to the causal foundation of the descent 

 theory. The doctrine of causes which has brought about the change of 

 species forms the nucleus of the Darwinian theory, by which it is especially 

 distinguished from Lamarckism. In order to substantiate causally the 

 change of species, Darwin proposed his highly important principle of 

 'Natural Selection by means of the Struggle for Existence.' 



