HISTORY OF ZOOLOGY. 43 



is an island zone between the two in which a mixture of faunas 

 occurs. Celebes especially belongs here. (3) A long time before 

 Darwin, the renowned geologist Leopold von Buch, from the dis- 

 tribution of plants on the Canary Islands, had come to the conclu- 

 sion of a change of species into new species; viz., on islands 

 peculiar species develop in secluded valleys, because high mountain- 

 chains isolate plants more effectually than do wide areas of water. 

 M. Wagner has collected many instances which prove that locali- 

 ties inhabited by certain species of beetles and snails have been 

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

 neighboring regions related so-called ' vicarious species ' are found. 



Causal Foundation of the Theory of Evolution. The Dar- 

 winian theory, so far as the above exposition shows, is fundamen- 

 tally like the theories of descent advocated at the beginning of this 

 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 over- 

 thrown by the type theory, and replaced it by the branched, tree- 

 like mode of arrangement, the genealogical tree. But still more 

 important are those advances of Darwinism 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. ' 



Artificial Selection. In the development of this principle 

 Darwin started from the limited and hence easily comprehended 

 subject of Domestication, the artificial breeding of our races of 

 domesticated animals. Many of these undoubtedly sprang from a 

 .single wild living species; others arose from several species, but 

 now have the appearance of a single species. How have arisen 

 .such extraordinarily different rapes of pigeons the fantail, the 

 pouter, long- and short-billed pigeons, etc., the long- and short- 

 horned cattle, the heavy, slow Percherons and the slenderly-built, 

 fleet-footed Arabian horses ? Undoubtedly through that same 

 more or less conscious influence of man, which is still employed 

 "by the skilful animal-breeder. If he wish to obtain a particular 

 form, he chooses from his stock suitable animals, which he pairs 

 together if they in ever so slight a manner approach nearer than 

 the others to the desired ideal. By repetition of this selection 

 according to plan, the breeder attains a slow but sure approx- 



