106 EDMUND B. WILSON 



when we must take into account more fully than has yet been done 

 the new complexities and possibilities that have continually been 

 unfolded as we have made better acquaintance with the chromo- 

 somes. In this respect the advance of cytology has quite kept 

 pace with that of the experimental study of heredity; and it has 

 established so close and detailed a parallelism between the two 

 orders of phenomena with which these studies are respectively 

 engaged as to compel our closest attention. 



Studies on the chromosomes have steadily accumulated evi- 

 dence that in the distribution of these bodies we see a mechanism 

 that may be competent to explain some of the most complicated 

 of the phenomena that are being brought to light by the study 

 of heredity. Xew and direct evidence that the chromosomes 

 are in fact concerned with determination has been produced by 

 recent experimental studies, notably by those of Herbst ('09) 

 and Baltzer ( J 10) on hybrid sea-urchin eggs. But the interest 

 of the chromosomes for the study of heredity is not lessened, as 

 some writers have seemed to imply, if we take the view it is hi 

 one sense almost self-evident that they are not the exclusive 

 factors of determination. Through then" study we ma}' gain an 

 insight into the operation of heredity . that is none the less 

 real if the chromosomes be no more than one necessary link in a 

 complicated chain of factors. From any point of view it is 

 indeed remarkable that so complex a series of phenomena as is 

 displayed, for example, in sex-limited heredity can be shown to 

 run parallel to the distribution of definite structural elements, 

 whose combinations and recombinations can in some measure 

 actually be followed with the microscope. Until a better expla- 

 nation of this parallelism is forthcoming we may be allowed to hold 

 fast to the hypothesis, directty supported by so many other data, 

 that it is due to a direct causal relation between these structural 

 elements and the process of development. 



A second point that may be emphasized is the remarkable con- 

 stancj' of the chromosome-relations in the species, and their no 

 less remarkable plasticity in the higher groups. The scepticism 

 that has been expressed in regard to constancy in the species finds, 

 I think, no real justification in the facts. It is perfectly true that 



