CHAPTER IV 



THE REPRODUCTION AND LIFE-HISTORY OF 



ANIMALS 



I. Reproduction 



In the higher animals the beginnings of individual hfe 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 recogni- 

 tion 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 w^hich 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. 



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

 clearness in regard to sexual reproduction, because the pro- 

 cess 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 union or amphimixis of two different kinds of germ 

 cells, ova and spermatozoa, which come to nothing unless 

 they unite. Furthermore, these dimorphic reproductive 

 cells are produced by two different kinds of individuals 

 (females and males), or from different organs of one 

 individual, or at different times within the same organ 

 (hermaphroditism). 



It is conceivable that organisms might have gone on 



multiplying asexually, by detaching overgrown portions of 



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