The Problem of Adaptation 15 
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 use- 
fulness 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 (Lumébriculus) 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 differ- 
ent animals of this power of regeneration, but it may be stated, 
