It has already been pointed out that it appears to be almost a _reductio ad absurdum_ to speak of animals adapting themselves 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 disuse 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. Nevertheless, it seems to be true in the case of morphine and of arsenic, and probably for some other substances as well, that if their use is suddenly stopped the individual may die in consequence. In this respect the organism behaves exactly as it does to an environment to which it has become adapted. Regeneration Many animals are able to replace lost parts, and all of them can heal wounds and mend injuries. This power is obviously of great advantage to them, and it has been supposed by Darwin, and more especially by his followers, that the power has been acquired through natural selection. It is not difficult to show that regeneration could not, in many cases, and presumably in none, have been acquired in this way. Since I have treated this subject at some length recently in my book on “Regeneration,” I shall attempt to do no more here than indicate the outline of the argument. The Darwinians believe that, if some individuals of a species have the power to replace a part that is lost better than have other individuals, it would follow that those would survive that regenerate best, and in this way after a time the power to regenerate perfectly would be acquired.