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 an evident fact, that almost every 

 organism, whether plant or animal, arises from an egg-cell 

 or ovum which has been fertilised by a male-cell or sperm- 

 atozoon. 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. 

 We can easily see this simple beginning in the frog-spawn 

 from the ditch, in the eggs of salmon, in those of the pond- 

 snail {Lymnceus), and in hosts of other cases. 



History. — Perhaps we can realise this discovery better if we con- 

 sider its history. For a long time, on into the present century, what 

 was called the doctrine of preformation prevailed. According to this 

 theory, development was merely an unfolding ("evolution") of a pre- 

 formed miniature which lay within the germ. The " oviSts " found this 

 nliniature model of the future organism in the egg ; the " animalculists " 

 found and even figured it within the spermatozoon. "There is no 

 becoming," said Haller, "no part of the body is made from another, all 

 are created at once." But this was not all. The germ was more than 



