The Bearing of Cytological Research on Heredity. 351 



I have endeavoured to show how the chromosome-theory, first outlined in 

 very general form, has been more and more specifically developed until it 

 has become an important instrument for the detailed analysis of intricate 

 genetic phenomena. I am well aware that some eminent students of 

 genetics are still reluctant to accept this theory, at least in its more detailed 

 applications. I am not disposed to reproach them for such scepticism. The 

 cytologist suffers under the disadvantage of working in so unfamiliar a field 

 that some of his conclusions, even among those most certainly established 

 and most readily verifiable, ai*e apt to give a certain impression of unreality, 

 even to his fellow naturalists. It is undeniable, too, that in this subject, for 

 better or for worse, hypothesis and speculation have continually run far in 

 advance of observation and experiment. It is quite possible that some of my 

 hearers may consider some of the views I have touched upon as a fresh 

 illustration of this fact. If so, I beg them to bear in mind that no 

 conclusion which I have considered has been reached as a merely logical or 

 imaginative construction. I have endeavoured to limit myself to matters of 

 observed fact, and to conclusions that are either demonstrated by facts or 

 directly and naturally suggested by them. 



To those who have had opportunity to come into intimate touch with both 

 cytological and genetic research the conclusion has become irresistible that 

 the chromosomes are the bearers of the " factors " or " gens," with the 

 investigation of which genetics is now so largely occupied. What are these 

 gens ? How do they operate ? "We do not know what they are. We assume 

 only that a gen is something that is necessary to the development of a 

 particular character. We do not know how they operate ; for, despite all 

 that experimental cytology and embryology have taught us concerning 

 development, we are still without adequate understanding of its mechanism. 

 We may nevertheless guess that gens play their several roles by virtue of 

 their specific chemical nature, and that the study of chemical physiology as 

 applied to development is destined to take an important part in the future 

 investigation of this problem. In the meantime it would be well to drop the 

 term "determiner" or "determining factor" from the vocabulary of both 

 cytology and genetics. What we really mean to say is " differential " or 

 ~ ; differential factor," for it has become entirely clear that every so-called 

 unit character is produced by the co-operation of a multitude of determining 

 causes. Embryologists long since demonstrated by direct experiment that 

 the cell-protoplasm as well as the nucleus is concerned in the determination 

 of development. Our whole study of the cell leads us to the conclusion that 

 it is an organic system, in the operation of which no single element can be 

 wholly dissociated from the rest. When, therefore, we speak of nuclei or 



VOL. LXXXVIII. — B. 2 I) 



