VARIETY AND UNITY IN LIFE 



21 



terminated by such parasites. Fill a bottle with flies. All in 

 time will die of suffocation, but a certain few will outlast the 

 many. Bring in a number of wolf cubs. Some will become 

 relatively tame some will remain wolves, and between the 

 most fierce and the most docile we shall find all ranges of 

 variation. "What is one man's food is another man's poison." 

 This proverb is a recognition of the principle of individuality 

 which accompanies everywhere the formation of species, and 

 being everywhere present, it must be an integral part of the 



FIG. 11. Silver fox, Vulpes pennsylranicus argentatus. (Photograph 



by W. K. Fisher.) 



process. Such differences are not matters of structure alone. 

 Psychological differences, differences in instinct, in adapta- 

 bility, in rate of nerve processes are just as marked as differ- 

 ences in anatomy. They may separate one species from an- 

 other. They may be just as decided within the limits of the 

 species itself. Moreover, the beginning of variation is not with 

 the individual organisms. No two cells are absolutely alike, 

 and in the variance of the germ cells, from which individuals 

 spring, all the elements of their future variation are involved. 

 Without further discussion, it is evident that variety in life 

 is a factor in the history of our globe, that it may be expressed 

 in terms of number of species, but that the actual range of varia- 

 tion is far greater than the number of species, and that if causes 

 are to be judged by range of effects, we must find in the origin of 

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