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 preceding, 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 individual 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 structure has been altered in certain respects at least. The form of the individual remains the same as before, but so far as its powers of resistance are concerned it is a very different being.