378 Evolution and Adaptation 
the selection stopped the race would sink back to the former 
condition. 
All this touches only indirectly the main point that we 
have under consideration, namely, the existence of this 
power of resistance in cases where it cannot have been the 
result of any educative process. Since the responses to 
new poisons do not appear to be in principle different from 
the responses to those to which the organism may have pos- 
sibly been subjected at times in the past, we shall probably 
not go far wrong if we treat all cases on the same general 
footing. Whether the power of adaptation to certain sub- 
stances, such as nicotine, morphine, cocaine, arsenic, alcohol, 
etc.,is brought about by the formation of a counter-substance 
is as yet unproven. And while it seems not improbable 
that in some of these instances it may turn out that this is 
the case, especially for poisons of plant origin, it is better 
to suspend judgment on this point until each case has been 
established. 
_ In recent years it has been shown that the animal body 
has the power of making counter-substances when a very 
large number of different kinds of things are introduced 
into the blood. We seem to be here on the threshold of a 
field for discovery which may, if opened up, give us an in- 
sight into some of the most remarkable phenomena of adap- 
tation shown by living things. 
It has already been pointed out that it appears to be almost 
a reductio ad absurdum to speak of animals adapting them- 
selves to poisonous substances. It is curious, too, that in 
man at least the use of these substances may arouse a craving 
for the poison, or at any rate the individual may become so 
dependent on the poison that the depression following its dis- 
use may lead to a desire for a repetition of the dose. The 
two questions that are raised here must be kept apart, for 
the adaptation of the individual to the poison and the so- 
called craving for it may depend on quite different factors. 
