3O CM. CHILD. 



less highly differen elated than the tissues of the gametophyte 

 from which the gametes arise. The plants therefore are no excep- 

 tion to the general law that asexual reproduction occurs in the 

 less highly differentiated, younger organism or part and sexual 

 reproduction in the older, more highly differentiated. 



It has been pointed out above that the gametes are both mor- 

 phologically and physiologically among the most highly differ- 

 entiated cells that we know. This is exactly what might be 

 expected if they arise as parts of old and highly differentiated 

 organisms. Moreover, on any other basis it is very difficult to 

 account for the differentiation of the gametes, except teleo- 

 logically. A cell consisting of "undifferentiated germ plasm" 

 is certainly capable of forming a new organism at once, provided 

 nutritive material is accessible. Why does not the primitive 

 germ cell use the nutritive material which comes to it for such 

 development rather than for a series of complex differentiations 

 which disappear as soon as the development of the new organism 

 begins? To assert that these differentiations are connected 

 with the necessity of fertilization is to put the cart before the 

 horse. We know that many plants can be bred asexually for 

 an indefinite number of generations and some have even lost the 

 capacity for sexual reproduction or never possessed it. More- 

 over, recent experimental work has demonstrated that in some 

 infusoria the supposed necessity for periodic conjugation is a 

 myth and that under proper conditions these animals can be 

 bred asexually for thousands of generations and perhaps in- 

 definitely. In experiments of my own with planarians I have 

 already bred the animals asexually for twelve generations and 

 during this time there has been no loss of vigor and functional 

 sexual organs have not developed at any time. It seems much 

 more nearly in accord with the facts to conclude that fertilization 

 is simply a necessary consequence of the differentiation of the 

 gametes as parts of the organism, rather than to maintain that 

 the differentiation of the gametes is a preparation for fertilization. 

 Besides this, it is difficult if not impossible to exclude the principle 

 of finality from the latter view. And lastly, the sexual differ- 

 entiation of the gametes which is characteristic of the higher 

 organisms is not necessary for fertilization since in many cases- 



