158 THE AMERICAN NATURALIST [Vol.XLIX 



marily into two groups, which we distinguish as somatic- 

 cells and germ-cells respectively. 



From this point onwards evolution ceases to be a 

 really continuous process, but is broken up into a series 

 of ontogenies, at the close of each of which the organism 

 has to go back and make a fresh start in the unicellular 

 condition, for the somatic cells sooner or later become 

 exhausted in their conflict with the environment and 

 perish, leaving the germ-cells behind to take up the run- 

 ning. That the germ-cells do not share the fate of the 

 somatic cells must be attributed to the fact that they take 

 no part in the struggle for existence to which the body 

 is exposed. They simply multiply and absorb nutriment 

 under the protection of the body, and therefore retain 

 their potential energy unimpaired. They are in actual 

 fact, as is so often said, equivalent to so many protozoa, 

 and, like the protozoa, are endowed with a potential 

 immortality. 



We know that, if placed under suitable conditions, or 

 in other words, if exposed to the proper environmental 

 stimuli, these germ-cells will give rise to new organisms, 

 like that in the body of which they were formerly en- 

 closed. One of the necessary conditions is, with rare 

 exceptions, the union of the germ-cells in pairs to form 

 zygotes or fertilized ova; but I propose, in the first in- 

 stance, for the sake of simplicity, to leave out of account 

 the existence of the sexual process and the results that 

 follow therefrom, postponing the consideration of these 

 to a later stage of our inquiry. I wish, moreover, to 

 make it quite clear that organic evolution must have 

 taken place if no such eyent as amphimixis had ever 

 occurred. 



What, then, may the germ-cells be expected to do? 

 How are they going to begin their development? In 

 endeavoring to answer this question we must remember 

 that the behavior of an organism at any moment de- 

 pends upon two sets of factors — the nature of its own 

 constitution on one hand, and the nature of its environ- 



