52 GENERAL PRINCIPLES OF ZOOLOGY. 



beginning of this century by Lamarck and other zoologists ; 

 it is distinguished from these only by its much more com- 

 prehensive empirical foundation, and further in that it 

 abandoned the successional arrangement laid down by the 

 type theory, and replaced it by the branched, treelike 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, whereby it is 

 especially distinguished from Lamarckism. In order to 

 substantiate causally the change of species, Darwin- pro- 

 posed his highly important doctrine of "Natural Selection 

 by means of the Struggle for Existence" 



Artificial Selection. In the development of this doc- 

 trine Darwin started from the limited and hence easily 

 comprehended subject of Domestication, the artificial breed- 

 ing of our races of domesticated animals. Many of our 

 domesticated animals sprang undoubtedly from a single 

 wild living species ; others originated from several species, 

 but now have the appearance of a single species. How 

 have arisen such extraordinarily different races of pigeons 

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

 the long- and short-horned cattle, the heavy, slow Percher- 

 ons and the slenderly-built, fleet-footed Arabian horses? 

 Undoubtedly through that same more or less conscious in- 

 fluence of man, which is still employed by the animal-breeder 

 who pursues a purpose. If he wishes 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 sought-for ideal. By repeti- 

 tion of this selection according to plan, the breeder at- 

 tains a slow but sure approximation to the desired goal, 

 since from each new generation he uses for breeding only 

 the suitable individuals. If he wishes, for example, to 

 breed fan-tail pigeons, he selects from his stock animals 

 with the greatest possible number and strongest developed 

 tail-feathers. In the course of generations, then, charac- 



