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 
