SYMBIONTICISM IN RELATION TO HEREDITY 129 



specialized cell were dependent upon a microsymbiont, 

 then the differentiation of the speciaUzed cell in ontogenetic 

 development must, primarily, be dependent upon the 

 microsymbionts (mitochondria) associated with that cell. 

 In a recent paper, Davenport ('25) has attempted to 

 correlate the hereditary variations dependent upon chromo- 

 somal behavior w^ith those produced by endocrine distur- 

 bances. After directing attention to the hereditary simi- 

 larities produced by these two methods, he says: 



How can the difference between the views of the geneticist and 

 the chromosomologist, on the one hand, and the endocrinologist, 

 on the other, be reconciled? It would appear that both views 

 can not be true, provided they are mutually exclusive. If the 

 chromosomes are alone responsible for resemblance then the 

 endocrinologists must be deceived in their conclusion. If the 

 hormones are alone responsible for resemblance then there must 

 be something false in the scientific methods of the geneticist. 

 Either of these conclusions is, however, untenable since both the 

 students of chromosomes and of hormones have worked by the 

 best of scientific methods so that their results are unassailable. 

 Hence we must consider both views to be true and that the chromo- 

 somes and the hormones each have their role to play in the direc- 

 tion of development. The most tenable hypothesis of the nature 

 of the chromosomes is indeed that they are packages of enzymes 

 which activate the metabolic processes of the early stages of 

 development, just as the hormones of the endocrine glands control 

 metabolism in later stages. The hypothesis may be suggested 

 that the chromosomes direct the early stages of development and 

 create certain centers of chemical activity to which they hand 

 over the business of differentiation of particular parts. Thus the 

 chromosomes may work indirectly in estabhshing certain centers 

 of later chemical activity whose course they have determined, 

 but in the working of whose mechanism they subsequently do not 

 interfere. The endocrine glands of vertebrates represent perhaps a 

 still later and highly specialized stage in the series of regulators of 

 metabolism. In the invertebrates and in plants where such endo- 



