ANIMAL PEDIGREES 245 



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 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 mor- 

 phologically 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 neces- 

 sary before 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 

 plants devices of the most varied and even extra- 

 ordinary kind are adopted to ensure that such 

 cross-fertilisation occurs. The essence of the act 

 of cross-fertilisation, which is already established 

 among Protozoa, consists in combination of the 

 nuclei of two cells, male and female, derived from 

 different individuals. The nature of the process is 

 of such a kind that two individual cells are alone 

 concerned in it ; and it may I think be reasonably 

 argued that the reason why animals commence 

 their existence as eggs i.e., as single cells is 

 because it is in this way only that the advantage 



