

LOTSY, CURRENT THEORIES OF EVOLUTION. 395 
blue is not caused by one of its constituents but by the interaction 
of them all. 
This illustrates perhaps better than any other case I can think 
of, the fundamental difference between the point of view of the 
gene-conception based on mere morphological consideration and 
the physico-chemical conception based on physiological conside- 
rations. Whatever the structure of an organism may be it certainly 
is the seat of very complicated chemical interactions, which, the 
persistance of organisms of a certain kind, during long geological 
periods, proves it, are in all essentials independent from such 
changes in the external circumstances as happen to occur in nature. 
To assume that these interactions take place in essentially the 
same way, generation after generation, and then should suddenly 
change, without any apparent cause, seems to me an unwarranted 
assumption which comes dangerously near to the doctrine of creation. 
And here we have reached the point where the morphological (gene) 
and the physiological (physico-chemical) conception can never agree 
as to the significance of changes occurring in heterozygous organisms. 
While the morphologists may, by accumulation of considerable 
circumstantial evidence, such as for instance that obtained by 
MORGAN in the case of Drosophila, even if such evidence ts obtained 
from a heterozygous organism, feel finally convinced, that a gene 
has mutated, the physiologist, in the sense as explained above, can 
never ascribe mutational value to a change obtained from heterozy- 
gous stock, no matter how wide a meaning one is willing to give 
to this term short of a change caused by the insertion of something 
previously foreign to the stock in question. 
For him, just as for the chemist, purity of the initial material 
is a conditio sine qua non for the obtention of definite proof 
of the existence of mutation. 
In heterozygous organisms the possibilities of interaction of 
chromosomes differing in structure may simulate mutation, may cause 
a change in a particular molecular group situated at a particular 
point in one of the chromosomes, for, to my mind also there is no 
doubt that chromosomes and cytoplasm both have a definite molecular 
structure, but this is not mutation but simply chemical interaction 
between two objects capable to enter into such an action with one 
another. Such a change could only then be interpreted as a mutation 
