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. New 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 (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 in 
one sense almost self-evident—that they are not the exclusive 
factors of determination. Through their study we may 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, directly 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- 
stancy 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 
