midwife toad (_Alytes obstetricans_) the male carries the eggs about attached to his legs, respiration is effected by enlarged external gills, and the larvae are hatched in water. In the ancestral reptiles external gills may have helped at first, until by the enlargement of the bladder they were rendered unnecessary. In all such cases the absorption of oxygen must be regarded as the stimulus which caused the enlargement of the respiratory membrane. As the allantois could not be absorbed or retracted again into the abdomen, the umbilicus was evolved--that is to say, the scar formed by the union of the folded edge between the body wall and amnion surrounding the stalk of the allantois. It would he difficult for a mutationist to explain how a mutation should affect the development of the cloacal bladder to such an enormous degree, just when it was required for embryonic respiration, and cause the sides of the body to unite ventrally at the time of hatching, cutting off the allantois and the amnion. T. H. Morgan [Footnote: _A Critique of the Theory of Evolution_, p.18.] states that a mutation of gametic origin may affect any stage in the development of the individual. This may be true when there are already distinct stages in the life history. The more important question is whether distinct stages can be caused by mutation. It is true that in heterozygous individuals characters may develop more fully in the adult stage than in the young. But when we find different stages evidently adapted to different modes of life, it is impossible to explain them by mutations affecting different stages of life. In such cases as the larval stages of Insects we find the larvae have become adapted to new habits while the adults have remained unchanged, or have evolved quite independent adaptations. For example, the adults in the chief orders of Insects have the typical three pairs of legs, while the maggots or grubs of the Diptera or Hymenoptera have no legs at all, the caterpillars of Lepidoptera have evolved pseudo-legs on the abdomen, and the larvae of Coleoptera have the ordinary legs and no more. This is the reverse of recapitulation: in the case of legless maggots, and caterpillars with pro-legs, the adult is more similar to the ancestor than the larva. But the same principle holds, that where functions and habits are different, there organs are different. No mutationist has yet produced by breeding experiments a caterpillar without the three pairs of thoracic legs and yet developing into a moth that had normal three pairs. Morgan, with all his mutations of the adult _Drosophila_, says nothing of mutants possessing legs. The only rational conclusion is that legless larvae have lost the disuse, since those larvae which are destitute of legs do not go in search of food but either live in the midst of it or are fed by others, and that the pro-legs of the caterpillar have been developed by the muscular action of the insect in clinging to leaves.