In lizards, a leg which has been cut off does not grow again, but an amputated tail does, and this has quite a definite biological reason, since the active little animal will seldom be caught by the foot by any pursuer, but may easily be caught by the tail, which is far behind. Thus the tail is adapted not only for regeneration, but also for 'autotomy' that is, for breaking off easily when it is caught hold of. We have already seen that some segmented worms have a very high regenerative capacity; yet every part cannot produce every other, and while, in _Lumbriculus_, any piece of from five to nine segments is able to grow a new head or tail, neither ten nor twenty nor all the segments together, if they are _halved longitudinally_, can reproduce the other half, and the cause of this inability does not lie in the fact that the animal is thereby hindered from taking food, for even the transversely cut pieces do not feed until they have grown a new head and tail. The reason must lie in the fact that the primary constituents for this kind of regeneration are wanting, and they are so because a longitudinal splitting of this cylindrical and relatively thin animal never occurs under natural conditions, and thus could not be provided against by Nature[1]. [1] Morgan maintains that this statement is incorrect, and that _Lumbriculus_ is capable of lateral regeneration. But if we look into the matter more closely we find that all he says is, that small gaps made by cutting a piece out of one side are filled up again, while the cut pieces perish. If the whole animal be halved, according to Morgan, both halves die, or if a 'very long piece' be cut out of one side, not only this piece dies, but also 'the remaining piece.' There is thus, as I have said, an essential difference between the regenerative capacity of _Lumbriculus_ and that of _Planaria_.