244 ANIMAL PEDIGREES 



not be given at present, but a partial one may 

 perhaps be obtained. Darwin himself suggested 

 that the clue might be found in the consideration 

 that at whatever age a variation first appears in the 

 parent, it tends to reappear at a corresponding age 

 in the offspring ; but this must be regarded rather 

 as a statement of the fundamental fact of embry- 

 ology than as an explanation of it. It is probably 

 safe to assume that animals would not recapitulate 

 unless they were compelled to do so : that there 

 must be some constraining influence at work, 

 forcing them to repeat more or less closely the 

 ancestral stages. It is impossible, for instance, to 

 conceive what advantage it can be to a reptilian or 

 mammalian embryo to develop gill-clefts which are 

 never used, and which disappear at a slightly later 

 stage ; or how it can benefit a whale, that in its 

 embryonic condition it should possess teeth which 

 never cut the gum, and which are lost before birth. 

 Moreover, the history of development in different 

 animals or groups of animals offers to us, as we 

 have seen, a series of ingenious, determined, varied, 

 but more or less unsuccessful efforts to escape 

 from the necessity of recapitulating, and to sub- 

 stitute for the ancestral process a more direct 

 method. 



A further consideration of importance is that 

 recapitulation is not seen in all forms of develop- 

 ment, but only in sexual development ; or at least 

 only in development from the egg. In the several 

 forms of asexual development, of which budding 

 is the most frequent and most familiar, there is no 



