106 EVOLUTION AND ANIMAL LIFE 



. The role played by the phenomena of transmission as distin- 

 guished from that of heredity has never been clearly ascertained. 

 Many eminent writers ascribe to it a large importance. It is 

 a central element in Mr. Casper RedfiekTs theory of heredity, 

 and he brings together a considerable array of facts and statis- 

 tics to justify his conclusions. But the value of statistics in 

 such matters is easily exaggerated, because of the difficulty in 

 ascertaining the real causes behind the phenomena we try to 

 record. It is fair to say as a broad proposition that, as a sound 

 mind requires a sound body, soundness both of mind and body 

 are factors in giving to offspring the best possible start in life. 

 The heredity unchanged, there is still a great value in vigor of 

 early development. 



The relation of these matters to the theory of organic evo- 

 lution is mainly here: artificial selection as a process is of the 

 same general character as natural selection; both represent a 

 form of isolation or segregation, which prevents indiscriminate 

 mating, and which holds certain groups of individuals as the 

 agents of reproduction of the species within a given time or in 

 a special area. 



Artificial selection intensifies useful or adaptive characters, 

 using these words in a broad sense. At the same time, it per- 

 petuates a series of characters, in no wise useful, and in no 

 fashion adaptive. The,3e characters remain unchanged for long 

 periods, and hence have more value in race distinction or in 

 classification than the strictly adaptive characters have. A 

 Southdown sheep is plump and fat, on the whole perhaps more 

 so than any other type of sheep. Nevertheless, it is not by its 

 plumpness that we know a Southdown. It is rather by the 

 character of its wool, the color of its face and feet, the form of 

 its head. So it is with breeds and races generally. They are 

 formed primarily by isolation in breeding, the separation of a 

 few from the many by geographical or similar causes, by the 

 perpetuation of the traits of these few (the "survival of the 

 existing"), all this being modified by the new range of natural 

 and artificial selection and the new reactions under the varying 

 conditions of a new environment. 



It interests us to know that a similar process takes place in 

 nature. Geographical and topographical barriers are crossed 

 in migration. These isolate a portion of a species under new 

 conditions, with new reactions to the environment, and a 



