RECAPITULATION 231 



another of those cases in which the function to which 

 structure is adapted is constant from the beginning 

 of independent life to the end, and there is some 

 ground for believing that in course of time in such 

 cases embryonic recapitulation may be much 

 diminished or disappear. The period of time since 

 birds were first evolved is in all probability im- 

 mensely greater than that which has elapsed since 

 the blind fish, Amblyopsis, was modified by cave- 

 life, so that we can understand why the eye is de- 

 veloped to a certain stage in the embryo of the blind 

 fish, although it lives in darkness all its life, while 

 embryonic recapitulation in the wing of the bird is 

 very incomplete. 



In another class of adaptations the embryonic or 

 larval stage is adapted to new conditions, while the 

 adult condition is either less changed or not changed 

 at all. One of the most obvious examples of this 

 is the allantois in the Amniota. The embryos of 

 Reptiles, Birds, and Mammals all develop two 

 embryonic or foetal membranes, the amnion and 

 the allantois. Of the function or origin of the 

 amnion little is known : to state that it is protective 

 affords little explanation. It seems possible that 

 it is merely the mechanical result of the weight of the 

 embryo and the development of the allantois. The 

 latter is a precocious hypertrophy of the cloacal 

 bladder found in Amphibia, with the function of 

 embryonic respiration. In the water the amphibian 

 larva respires by means of gills and gill slits. In 

 adaptation to terrestrial life it is necessary, if the 

 free aquatic larval stage is to be eliminated, that the 

 embryo should be able to breathe air before hatching. 

 Various Amphibia show how this requirement was 



