PRESIDENTIAL ADDRESS. 43 



Darwin himself suggested that the clue might be found in the con- 

 sideration 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 

 embryology 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 con- 

 straining 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 substitute for the ancestral 

 process a more direct method. 



A further consideration of importance is that recapitulation is not 

 seen in all forms of development, 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 repetition of ancestral phases ; neither is there 

 in cases of regeneration of lost parts, such as the tentacle of a snail, 

 the arm of a starfish, or the tail of a lizard ; in such regeneration it 

 is not a larval tentacle, or arm, or tail, that is produced, but an 

 adult one. 



The most striking point about the development of the higher animals 

 is that they all alike commence as eggs. Looking more closely at the 

 egg and the conditions of its development, two facts impress us as of 

 special importance : first, the egg is a single cell, and therefore 

 represents morphologically the Protozoan, or earliest ancestral phase ; 

 secondly, the egg, before it can develop, must be fertilised by a 

 spermatozoon, just as the stimulus of fertilisation by the pollen grain 

 is necessary before the ovum of a plant will commence to develop 

 into the plant embryo. 



