The Making of Species 
Many variations which organisms display are 
of a mixed kind, being in part the result of inner 
forces and in part due to the action of the en- 
vironment. In so far as they are due to this 
latter they do not appear to be inherited. 
Thus, although we cannot say of many varia- 
tions whether they are germinal, or somatic, or 
of a mixed kind, it is of great importance to keep 
continually in mind the fundamental differences 
between the two kinds. 
Some somatic variations are due to the direct 
action of the environment; they are merely the 
expression of the manner in which an organism 
responds to external stimuli. 
What is the cause of germinal variations? 
This is a question to which we are not yet ina 
position to give a satisfactory answer, 
The attempt to explain their origin plunges us 
into the realm of theory. This doubtless is a 
realm full of fascination, but it is an unexplored 
region of extreme darkness, in which, we believe, 
it is scarcely possible to take the right road until 
more of the light of fact has been shed upon it. 
In the chapter dealing with inheritance we 
shall indicate the lines along which it is likely 
that future progress will be made. 
TIo 
