CHAPTER III. 
DIFFICULTIES INHERENT IN THE THEORY. 
“ Lest men suspect your tale untrue, 
Keep probability in view.”—Gay. 
THERE are certain difficulties in connection with this 
theory which meet us on the very threshold of the enquiry; 
inasmuch as they are inherent in the theory itself. This 
is certainly not what we should expect to find. 
In the first place, it strikes one as rather startling that 
the transmutation of species by means of Natural Selection 
can only come into action in the face of adverse changes. 
For this process of transmutation starts from the point 
where a species has become adapted to its external con 
ditions. Now if the conditions, though changing in detail, 
are nevertheless equally favourable to the race, it is obvious 
that no modification can be wrought by Natural Selection, 
for no change would then be useful to the race. If 
altered conditions were still more favourable than the old 
ones, there would be still less need for any responsive 
adaptation. It is, therefore, only in the face of adverse 
circumstances which make modification a necessity—a 
matter of life and death—that Natural Selection can 
come upon the scene. Disastrous change is the overture 
to the opera : the prologue to the play. 
These changes must not be too rapid or the organisms 
would perish : they must not be too mild or they would 
not involve a question of life and death, i.e., they would 
not bring Natural Selection into action. The theory