8 NATURE SKETCHES IN TEMPERATE AMERICA 



principle of inheritance the selected variety \v\\\ tend to 

 propagate its new and modified form. Darwin says: "This 

 preservation, in the battle of life, of varieties which possess 

 advantages in structure, constitution, or instinct, I have called 

 natural selection." Spencer has well expressed the same idea 

 by the survival of the fittest. 



Darwin placed emphasis on the point that natural selection 

 does not imply conscious choice. For brevity's sake he some- 

 times spoke of natural selection as an intelligent power in the 

 same way as astronomers speak of the attraction of gravity 

 as ruling the movements of the planets. Moreover, he often 

 personified nature, but he adds: "I mean by nature only the 

 aggregate action and product of many natural laws, and by 

 laws the ascertained sequence of events as ascertained by us." 



"Under domestication we see much variability caused, or 

 at least excited, by changed conditions of life. Variability is 

 governed by many complex laws, by correlated growth, com- 

 pensation, the increased use and disuse of the parts, ^ and the 

 definite action of the surrounding conditions, such as climate 

 and food suj)plies. There is no question but that our domestic 

 productions have been modified and these modifications have 

 been inherited for long periods. Variability is not actually 

 caused by man, but the organism is exposed to new conditions 

 of life, and nature acts on the organization and causes it to 

 vary." 



But man can and does select variations given him by nature 

 and thus accumulates them in any desired manner. "The 

 key is man's power of accumulative selection, nature gives 

 successive variations, man adds them up in certain directions 

 useful to him. It is the magician's wand by means of which 

 he may summon into life whatever form and mould he pleases." 

 Taking the domestic pigeon, for example, he was able to trace 

 its ancestry to the wild rock pigeon which is still extant. It is 

 certain that man can select individual differences in a breed 

 so slight as to be unappreciated except to an educated eye. 

 Man can produce a great result by his methodical and uncon- 

 scious means of selection. 



' The variations caused by increased use and disuse of parts are not now 

 considered by many investigators to be inherited. 



