THE RECAPITULATION THEORY 355 



the gum, and which are lost before birth. More- 

 over, 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 ancestrg__procss a more direct method. A 

 further consideration of importance is that recapitu- 

 lation is noTseerTin all forms of development, but 

 only in sexual development ; or at least only in 

 developmentTfrom' the egg. In the several forms 

 of asexual development, of which budding is the 

 most frequent and most familiar, there is no repeti- 

 tion 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 com- 

 mence 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 morpho- 

 logically the Protozoon, 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 be- 

 fore the ovum of a plant will commence to develop 

 into the plant-embryo. 



The advantage of cross-fertilisation in increasing 

 the vigour of the offspring is well known, and in 



