No. 542] SOME ASPECTS OF CYTOLOGY 



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selves also in respect to size, behavior, or both. In one 

 case only has it thus far been possible to demonstrate a 

 constant relation between particular chromosomes and 

 particular characters, namely in the case of sex and sex- 

 limited characters. It is true that in this case we are not 

 able to assert that the sex-chromosomes are the primary 

 determining cause of sex — indeed, there seems to be good 

 evidence to the contrary. Unless, however, we are pre- 

 pared to defend the proposition that the sex-ehvomosomes 

 are absolutely functionless we shall not, I think, escape 

 the conclusion that they form one of the factors in sex- 

 heredity. 



I will not enter upon the analytical subtleties of the 

 problem whether the chromosomes, or the substances 

 that they contain, are permanent and self-propagating 

 elements or are merely temporary products of an unseen 

 underlying activity — whether they are causes, effects or 

 mere accompaniments of the specific reactions with which 

 they are somehow connected. These are fundamental 

 questions; and some of them can not yet be answered. 

 But we should not hesitate to adopt what seems likely to 

 be for the time the most reasonable and fruitful working 

 view. ' ' Hypotheses, ' ' said Pasteur, * ' come into our labo- 

 ratories by armfuls ; they fill our registers with projected 

 experiments, they stimulate us to research— and that is 

 all." In my view studies in this field are at the present 

 time most likely to be advanced by adopting the compara- 

 tively simple hypothesis that the nuclear substances are 

 actual factors of reaction by virtue of their specific chem- 

 ical properties ; and I think that it has already helped us 

 to gain a clearer view of some of the most puzzling prob- 

 lems of genetics. But even if we adopt the opposite view 

 that the formation, segregation and distribution of these 

 substances are only signs or indices of what lies behind 

 them, we still have in this direction one of the most prom- 

 ising paths of approach to a study of the activities of the 

 germ-cells in heredity. 



It will perhaps be said that such conceptions of the 



