External and Internal Factors in Evolution 323 



It has been shown that certain rotifers and tardigrades, 

 and also some unicellular animals, that live in pools and 

 ponds that are liable to become dry, withstand desiccation, 

 while other members of the same groups, living in the sea, 

 do not possess this power of resistance. Cases of this sort 

 are usually explained as cases of adaptation, but it has not 

 been shown experimentally that resistance to drying can be 

 acquired by a process of acclimatization to this condition. 

 The case is also in some respects different from the preced- 

 ing, since intermediate conditions are less likely to be met 

 with, or to be of sufficiently long duration for the animal to 

 become acclimatized to them. It seems more probable, in 

 such cases, that these forms have been able to live in such 

 precarious conditions from the beginning because they could 

 resist the effects of drying, not that they have slowly acquired 

 this power. Finally, there must be discussed the question of 

 the acclimatization to poisons, to which an individual may be 

 rendered partially immune. The point of special importance 

 in this connection is that the animal may be said to respond 

 adaptively to a large number of substances, which it has 

 never met before in its individual history, or to which its 

 ancestors have never been subjected. It may become slowly 

 adapted to many different kinds of injurious substances. 

 These cases are amongst the most important adaptive indi- 

 vidual responses with which we are familiar, and the point 

 cannot be too much emphasized that organisms have this 

 latent capacity without ever having had an opportunity to 

 acquire it through experience. 



The preceding groups of phenomena, included under 

 the general heading of individual acclimatization, have one 

 striking thing in common, namely, that a physiological 

 adaptation is brought about without a corresponding 

 change in form, although we must suppose that the struc- 

 ture has been altered in certain respects at least. The form 

 of the individual remains the same as before, but so far 



