34 GENERAL PRINCIPLES OF ZOOLOGY 



tion with llic other continents. \Miile in the other parts of the carfli the 

 higher vertebrates, which were de\eloped from the marsupials and tlieir 

 lower contemporaries, came, by way of the kinds 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 preser\ed. 

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

 into an eastern antl a western half. The fauna of the lirst has a thoroughly 

 Australian clraracter; that of the latter recalls Farther India antl the 

 Oriental rro\ince. DilTerences in climate and vegetation cannot be the 

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

 moist climates, with sparse and \\ith lu.xuriant vegetation. The only ex- 

 planation is that the eastern Malay Islands have developed geologically 

 in connection with Australia, the western with India. WaUace 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 Celelies) in which a mixture of faunas occurs. (3) Long 

 before Darwin, the geologist von Buch, from the tlistribution 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 inhaliited by certain species of beetles and snails have been 

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

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

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

 mention. The Hawaiian Islands liave 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 lilce the 

 theories of descent advocated at the beginning of tlie last centur)' by 

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

 much more extensive founilation of facts, and further in tliat it abandoned 

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

 mode of arrangement — the genealogical tree. But still more important 

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

 theory. The doctrine of causes which has brouglit about tlie 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.' 



