CONCLUSION 



The clicinical point of view leads us naturally to the tliird com- 

 bination, that by wliich heredity and development are brought 

 together as aspects o£ the same problem of propagation and 

 organization in two types of structure, the nucleus and the 

 cytoplasm. 



These relations between nuclear genes and cytoplasmic proteins 

 prepare the way for yet another combination, namely that between 

 heredity and infection. By comparing the organization of the 

 nucleus and the chromosomes with that of the less coherent or 

 smaller systems outside or beneath them, we can put these different 

 disciplines into their correct relationships for the first time. 



The range of relationships of cytoplasmic determinants with the 

 nucleus and with one another, its paralleHsm with the relationships 

 of viruses, and finally the gradation that exists between viruses, 

 plasmagenes and other proteins, have shown that the distinctions 

 between these types of body are conditional rather than fundamen- 

 tally genetic or chemical. The legitimate inlieritance of the plasma- 

 gene, and the infection of the piratical protein which could become a 

 virus, can be combined, as we saw, in one body. The dichotomy 

 exists rather between different types of viruses. Here, then, genetics 

 combined with chemistry has provided the means of combination 

 of plant, animal and human pathology and has even shown what 

 is significant in the different types of origin and transmission of 

 cancer. 



Finally, there is that great union which has resolved the conflict 

 of biometry and mendeHsm, of continuity and discontinuity. This 

 union depends on the modern refinement of statistical theory. But 

 it is again related to the activities of the chromosomes, putting, as 

 it does, the difference between the appearances of continuous and 

 discontinuous variation into terms of differences in size and specificity 

 of the genes and of the proteins they produce. At the same time 

 this combination resolves the long-standing conflict between the 

 supposedly useless methods of mendelian analysis and the practical 

 methods of improvement of plants and animals by breeding. 



Around these central combinations are several peripheral ones of 

 great interest. Two are notable. First, the union of systematics and 

 the theory of descent with the study of chromosomes has already, 

 as we have seen, borne great fruit in flowering plants. Later it will 



368 



