338 CAUSES OF VARIATION 



course of embryonic development is involved in the mystery of 

 ordinary differentiation from totipotent or indifferent tissue ; but 

 regenerated matter which arises from tissue in no wise involved 

 in embryonic development would seem to be outside of the influ- 

 ence of heredity. In the latter case it may result in normal tis- 

 sue, as in the lens of the eye, or in heteromorphic growth, as in 

 the production of an antenna instead of an eye, or of a foot instead 

 of an antenna. 



SECTION XI EVIDENCE FROM DEVELOPMENT AND 

 DIFFERENTIATION J 



From the fertilized ovum to the fully differentiated adult in- 

 dividual of the highest species is a long step. Nothing is more 

 evident than that all this differentiation is the result of forces 

 resident within the single cell of the oviim. Whatever office is 

 discharged by outside agencies, and whatever deviations or modi- 

 fications are produced thereby, the impulse to differentiate and 

 the direction of differentiation arise from forces internal to the 

 organism. Moreover, this differentiation, great as it is in the 

 finished product, when traced backward merges by imperceptible 

 shades each into the next preceding, until the undifferentiated 

 ovum is reached at the end (or beginning) of the series. 



What is the nature of these internal forces, and what are the 

 agencies that set them in motion, are the chief mysteries in ani- 

 mal and plant development. That a single germ cell, similar in 

 its essential nature to any one of the tissue cells of which the 

 body is composed, that such a cell " can carry with it the sum 

 total of the heritage of the species, that it can in the course of 

 a few days or weeks give rise to a mollusk or a man, is the 

 greatest marvel of biological science." 2 



" In attempting to analyze the problems that it involves," con- 

 tinues Wilson, " we must from the outset hold fast to the fact 

 on which Huxley insisted, that the wonderful formative energy of 

 the germ is not impressed upon it from without, but is inherent 

 in the egg as a heritage from the parental life of which it was 

 originally a part. The development of the embryo is nothing new. 



1 Wilson, The Cell, p. 430. 2 Ibid. p. 396. 



