CHAPTER IV. 



THE REPRODUCTION AND LIFE HISTORY OF 

 ANIMALS. 



I. Reproduction. 



In the higher animals the beginnings of individual life are 

 hidden, within the womb in Mammals, within the egg-shell 

 in Birds. It is natural, therefore, that early preoccupation 

 with those higher forms should have hindered the recog- 

 nition of what seems to us so evident, that almost every 

 animal arises from an egg cell or ovum which has been 

 fertilised by a male cell or spermatozoon. The exceptions 

 to this fact are those organisms which multiply by buds or 

 detached overgrowths, and those which arise from an egg 

 cell which requires no fertilisation. Thus Hydra may form 

 a separable bud, much as a rose-bush sends out a sucker ; 

 thus drone-bees "have a mother, but no father," for they 

 arise from parthenogenetic eggs which are not fertilised. 

 Apart from these and similar cases, the " ovum theory," 

 which Agassiz called " the greatest discovery in the natural 

 sciences in modern times," is true — that each organism 

 begins from the division of a fertilised egg cell. 



Sexual reproduction. — There is apt to be a lack of clear- 

 ness in regard to sexual reproduction, because the process 

 which we describe by that phrase is a complex result of 

 evolution. It involves two distinct facts — (a) the liberation 

 of special germ cells from which new individuals arise ; {b) 

 the occurrence of two different kinds of germ cells, ova and 

 spermatozoa, which come to nothing unless they unite 

 (fertilisation). Furthermore, these dimorphic reproductive 

 cells are produced by two different kinds of individuals 



