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THE AMERICAN NATURALIST 



[Vol. XLIX 



stance of the zygote is partially modified in molecular 

 constitution, perhaps by the direct action of the environ- 

 ment, as appears to happen in the case of Tower's ex- 

 periments on mutation in the potato beetle, or by the in- 

 troduction of a different sample of chromatin from an- 

 other individual by hybridization. What is the germ- 

 plasm now going to do! When and how may the 

 changes that have taken place in its constitution be ex- 

 pected to manifest themselves in the developing or- 

 ganism? 



Let us consider what would be likely to happen in the 

 first stages of ontogeny. If the germ-plasm had re- 

 mained unaltered the zygote would have divided into 

 blastomeres under the stimuli of the same conditions, 

 both internal and external, as those under which the 

 corresponding divisions took place in preceding genera- 

 tions. Is the presence of a number of new colloidal mul- 

 timolecules in the germ-plasm going to prevent this! 

 The answer to this question probably depends partly 

 upon the proportion that the new multimolecules bear 

 to the whole mass, and partly upon the nature of the 

 modification that has taken place. If the existence of 

 the new multimolecules is incompatible with the proper 

 functional activity of the germ-plasm as a whole there 

 is an end of the matter. The organism does not. de- 

 velop. If it is not incompatible we must suppose that 

 the zygote begins its development as before, but that 

 sooner or later the modification of the germ-plasm will 

 manifest ftself in the developing organism, in the first 

 instance as a mutation. In cases of hybridization we 

 may get a mixture in varying degrees of the distinguish- 

 ing characters of the two parent forms, or we may get 

 complete dominance of one form over the other in the 

 hybrid generation, or we may even get some new form, 

 the result depending on the mutual reactions of the dif- 

 ferent constituents of the germ-plasm. 



The organism into which any zygote develops must be 



