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. 



