VARIATION AND ADAPTATION 215 



have the secret of the mystery, although the explanation may 

 not be complete in all details. The metamorphosis of Ambly- 

 stoma, and to a certain degree of all Amphibia, is partly due to 

 heredity, partly to conditions. Usually the conditions in which 

 the larvae live are such that, as summer advances, the water, 

 being in small shallow ponds or swamps, becomes reduced by 

 evaporation, raised in temperature, and for this reason as well 

 as from the presence of rotting vegetation deficient in oxygen : 

 the larvae are therefore forced to inhale air and so the develop- 

 ment of the lungs and the reduction of the gills are equally pro- 

 moted by the external conditions. There is also a hereditary 

 tendency towards this special response to the external conditions 

 with regard to oxygen. The structural changes are not directly 

 due to the change of conditions but the hereditary tendency is 

 called into action by those conditions. According to the pre- 

 sent writer's views, which are not shared by many other zoo- 

 logists, the facts are best explained on the theory that the 

 metamorphosis was in the first instance the direct result of the 

 change of conditions, repeated through many successive genera- 

 tions ; as time went on the tendency to the change of structure 

 became hereditary and thus the same change produced much 

 more result than in the beginning. Similar phenomena are 

 presented by many other cases of adaptive changes occurring 

 in development ; for instance, the assumption of the erect posi- 

 tion in the human species. A baby walks on all fours like a 

 monkey and has to learn to walk upright ; but it learns to walk 

 on the hind limbs with great rapidity with a little practice be- 

 cause there is a strong hereditary tendency to the requisite 

 structural changes. These changes, however, would not take 

 place so rapidly nor so completely by the action of heredity 

 alone, that is if the child were never allowed to practice equili- 

 brium and progression in the upright attitude. 



Now when in the case of amphibian larvae the changes in 

 external conditions with regard to oxygen do not occur the 

 hereditary tendency is not stimulated, and on the other hand 

 the functional stimulation of the gills and associated structures 

 is continued. This necessarily happens when the larvae live in 

 large lakes of which the water is always saturated with oxygen, 

 and it is in just such lakes that we find neoteny to occur. At 

 first the metamorphosis may only be postponed to a later period 



