When we consider that there are a number of bacterial diseases, in each of which a different poison is made by the bacteria, we cannot but ask ourselves if the animal really makes a counter-poison for each disease, or whether a single substance may not be manufactured that counteracts all alike? That the latter is not the case is shown by the fact that an animal made immune to one disease is not immune to others. When we recall that the animal has also the capacity to react in one way or another to a large number of organic and inorganic poisons, to which it or its ancestors can have had little or no previous experience, we may well marvel at this wonderful regulative power.
The healing of wounds, which takes place in all animals, forms another class of adaptive processes. The immense usefulness of this power is obvious when it is remembered how exposed most animals are to injuries. By repairing the injury the animal can better carry on its normal functions. Moreover, the presence of the wound would give injurious bacteria a ready means of entering the body. In fact, an intact skin is one of the best preventives to the entrance of bacteria.
Not only have most organisms the power of repairing injuries, but many animals have also the closely related power of regenerating new parts if the old ones are lost. If a crab loses its leg, a new one is regenerated. If a fresh-water worm (_Lumbriculus_) is cut into pieces, each piece makes a new head at its anterior end and a new tail at the posterior end. In this way as many new worms are produced as there are pieces. And while in a strict sense it cannot be claimed that this power of regeneration is of any use to the original worm, since the original worm, as such, no longer exists, yet since it has not died but has simply changed over into several new worms, the process is of use inasmuch as by this means the pieces can remain in existence.
We need not discuss here the relative importance to different animals of this power of regeneration, but it may be stated, that, while in some cases it may be necessary to replace the lost part if the animal is to remain in existence, as when a new head is formed on an earthworm after the old one was cut off, in other cases the replacement of the lost part appears to be of minor importance, as in the case of the leg of the crab. While we are not, for the moment, concerned with the relative importance of the different adaptations, this question is one of much importance in other connections and will be considered later.