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 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 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. 
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 ; (4) 
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 
