58 THEORY OF EVOLUTION 



Not only has this power to make whatever 

 combinations we choose great practical impor- 

 tance, it has even greater theoretical signifi- 

 cance; for, it follows that the individual is not 

 in itself the unit in heredity, but that within the 

 germ-cells there exist smaller units concerned 

 with the transmission of characters. 



The older mystical statement of the individ- 

 ual as a unit in heredity has no longer any in- 

 terest in the light of these discoveries, except 

 as a past phase of biological history. We see, 

 too, more clearly that the sorting out of factors 

 in the germ plasm is a very different process 

 from the influence of these factors on the devel- 

 opment of the organism. There is today no 

 excuse for confusing these two problems. 



If mechanistic principles apply also to em- 

 bryonic development then the course of devel- 

 opment is capable of being stated as a series 

 of chemico-physical reactions and the "indi- 

 vidual" is merely a term to express the sum 

 total of such reactions and should not be in- 

 terpreted as something different from or more 

 than these reactions. So long as so little is 

 known of the actual processes involved in devel- 



