526 THE THEORY OF EVOLUTION 



sand years than are seen in the " evolution " of domestic poultry 

 from the jungle-fowl and the Malayan fowl or aseel; or of the 

 many breeds of pigeons from the wild rock-pigeon which was 

 one of their most important if not the sole ancestor. Other 

 domestic animals tell the same story {cf. Figs. 283 and 284). 



In one respect, however, these domestic species differ from 

 wild species; they can usually be bred among themselves. Wild 

 species seldom can be crossed. This is not such a serious objection 

 as it might seem, in view of the physiological adjustments that 

 must exist in domestic as compared with wild animals. It is also 

 true that domestic animals are modified to suit the needs or fancies 

 of man, and not necessarily their own needs as are species in nature. 

 But they have been bred for this, as natural species are bred in 

 nature to meet the necessities of their lives or perish. Instead 

 of a defect in the evidence, this might be regarded as an element of 

 strength, since it shows that under one set of conditions one kind 

 of characters can arise by evolution, and, under other conditions, 

 characters of a different sort. 



Experimental Evolution. — The evolution that has occurred 

 in domesticated forms, as described in the preceding paragraphs, 

 has been brought about by the attempts of men to secure desired 

 types of animals and plants. The early men, who caught jungle- 

 fowls and penned them up, used the wilder ones as food or let 

 them escape, and thus the tamer individuals survived to breed. 

 In later times there was more conscious effort to improve the 

 breeds by crossing and by selecting only the desired types for 

 breeding. Thus, Darwin tells us that in Saxony during the early 

 nineteenth century men followed the breeding of sheep as a trade. 

 " The sheep," he says, " are placed on a table and are studied like a 

 picture by a connoisseur; this is done three times at intervals of 

 months, and the sheep are each time marked and classed, so that 

 the very best may ultimately be selected for breeding." 



In our own generation, breeders have effected great changes 

 and established new breeds. A conspicuous example among plant 

 breeders was Luther Burbank, who worked by crossing different 

 types of plants in large numbers and selecting, out of many 

 thousands, the ones that seemed most desirable. All such work 

 tends to become increasingly scientific. Knowledge of the Men- 

 delian laws of heredity and some of the fundamental facts con- 

 cerning variation enables the scientific experimenter to excel any 



