32 INTEODUCTION. 



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 Proto- 

 zoan, or earliest, ancestral stage ; secondly, the egg, before it 

 can develop, must, in the great majority of cases, 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. 



The advantage of cross-fertilisation in increasing the vigour 

 of the offspring is well known, and in plants devices of the 

 the most varied and even extraordinary kind are adopted to 

 ensure that such cross-fertilisation occurs. The essence of the 

 act of cross-fertilisation consists in combination of the nuclei of 

 two cells, male and female, derived from difierent individuals. 

 The nature of the process is of such a kind that two individual 

 cells are alone concerned in it; and it may reasonably be 

 argued that the reason why animals commence their existence 

 as eggs, i.e. as single cells, is because it is in this way alone that 

 the advantage of cross- fertilisation can be secured, an advantage 

 admittedly of the greatest importance, and to secure which 

 natural selection would operate powerfully. 



The occurrence of parthenogenesis in certain groups, either 

 occasionally or normally, is not so serious an objection to this 

 view as it appears at first. There are strong reasons for 

 holding that parthenogenetic development is a modified form, 

 derived from the sexual method. Moreover, it is the very 

 essence of the view advanced above, that it does not state that 

 cross-fertilisation is essential to individual development, but 

 merely that it is in the highest- degree advantageous to the 

 species ; and hence room is left for the occurrence, exceptionally, 

 of parthenogenetic development. 



It may be objected that this is laying too much stress on 

 sexual reproduction, and on the advantage of cross-fertilisation ; 

 but it must not be forgotten that sexual reproduction is the 



