FACTORS IN ONTOGENY 245 



assimilation, growth, differentiation, adaptation, heredity, 

 variation, etc., and it is also plain that these fundamental life 

 phenomena are to be most effectively studied in their relations 

 to the development of individual organisms. 



The most casual analysis of development shows that nu- 

 merous and various influences play their parts in determining 

 its course; it satisfies no one any longer to say that the course 

 and character of an animal's development is determined by 

 heredity. No influence or "force" of heredity can make up 

 in any degree in the case of the development of a chick, for 

 example, for the absence of a proper temperature. This purely 

 external factor of heat is as indispensable to the development of 

 the new chick creature as is the mysterious inherent capacity of 

 the tiny protoplasmic mass to unfold or change so radically 

 that it (and what it adds to itself) may become a peeping 

 chicken. And temperature is but one of a number of other 

 external factors that contribute to the creation of the new 

 chicken, as indeed the inherent capacity of the protoplasm 

 of a hen's egg cell to rearrange itself chickwise and no other 

 wise during development is but one among a number of neces- 

 sary intrinsic factors whose correlated influence or working is 

 part of the developmental mechanism. 



The influences or factors which determine the initiation, 

 course, and outcome of development, then, may be roughly 

 classified into intrinsic and extrinsic factors. And as in our 

 search for rational mechanical explanations of vital phenomena 

 we look on factors as causal, we may use the word "causes" 

 in place of "factors" or "influences" if we like. The intrinsic 

 causes we must believe to be dependent on or incident to the 

 protoplasmic structure of the germ stuff and to be largely the 

 guiding and determining factors in development, while the 

 extrinsic causes are largely such as supply stimulus and energy 

 for the development. Among intrinsic developmental factors 

 are included assimilation, growth, division, differentiation, 

 etc., all constituting what His calls the "law of growth"; under 

 extrinsic factors may be listed heat, light, moisture, food, 

 gravitation, osmosis, etc., composing, according to His, the 

 conditions under which the " law of growth " operates. 



In order to understand just what part each one of the vari- 

 ous developmental factors or causes plays, there is necessary 

 a most thorough analytical study of development, and an 

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