ANIMAL LIFE AND EVOLUTION 343 



another and to the establishment of divergent lines of this 

 movement, these changes in type and lines always being in 

 the direction of better adaptation or fitness to the conditions 

 of life. Several recent discoveries based upon the experi- 

 mental study of variation and heredity have much lessened 

 the importance of natural selection as a species-forming 

 agent. It still remains, however, the chief explanation of 

 adaptation. 



Artificial selection has much less in common with natural 

 selection than popularly supposed. It depends, however, as 

 natural selection does, upon the existence of variations and 

 heredity and upon the culling out of certain individuals which 

 are allowed to produce offspring while the great majority are 

 not so allowed. This culling out, or selection, however, is made 

 according to the needs and whims of man and not at all upon 

 the basis of natural advantage or fitness. As a matter of 

 fact most artificial selection is in the direction of unfitness 

 for existence under natural conditions. Very few domestic 

 races of animals could hold their own in nature. 



In artificial selection, man, by being able to control matings 

 exactly, has a means of modifying races much more rapidly 

 than they are usually modified in nature. Man not only selects 

 to live the individuals showing certain wished-for variations, 

 but can mate together individuals which show these particular 

 variations in highest degree. Or he can mate individuals 

 showing different variations which he would like to combine. 

 By taking advantage of the great present-day extension of our 

 knowledge of the facts and laws of variation, inheritance, and 

 selection the artificial breeder can work much more rapidly 

 and with much more accuracy than ever before. 



Segregation, or Isolation. The modification of species 

 either in nature, or under the hands of man, always includes as 

 part of the process the segregation, or isolation, from the whole 

 mass of individuals of a small number specially like each 

 other. This is effected in natural selection by the saving of the 

 few with advantageous variations and the death of the others; 

 in artificial selection, by the culling out process of the breeder. 

 These few saved or selected individuals breed together and are 



