276 ZOOLOGICAL PHILOSOPHY 



of individual to a tree or shrub or even to herbaceous perennials ; 

 for a tree, shrub, etc., is in reality a collection of individuals which hve 

 on one another, communicate together, and share a common hfe, in 

 the same way as the compound polyps of madrepores, millipores, 

 etc. ; as I have already proved in the first chapter of this second part. 



Fertilisation, the essential result of an act of sexual reproduction, 

 must be divided into two different kinds, one of which is higher or more 

 eminent than the other, since it belongs to the most perfect animals 

 (mammals). This comprises the fertilisation of viviparous animals, 

 while the other, which is inferior and less perfect, includes that of 

 oviparous animals. 



The fertihsation of viviparous animals immediately vivifies the 

 embryo exposed to it, and this embryo forthwith continues to live, 

 and feeds and develops at the expense of its mother, with which it 

 remains in communication up to birth. No interval is known be- 

 tween the act which prepares it for the possession of life and the 

 reception of life itself; moreover, this fertilised embryo is enclosed 

 in a membrane which contains no stores of food within it. 



The fertilisation of oviparous animals, on the other hand, only 

 prepares the embryo for the reception of life, but does not actually 

 confer life. Now this fertilised embryo of oviparous animals is en- 

 closed with a store of food in investments, which cease to communicate 

 with the mother before being separated from her ; and it only receives 

 life when a special factor, which may come sooner or later according 

 to circumstances, or may not come at all, communicates to it the vital 

 movement. 



This special factor, which confers life on the embryo of an oviparous 

 animal after it has been fertilised, consists as regards animals' eggs in a 

 mere rise of temperature, and as regards the seeds of plants in the co- 

 operation of moisture with a gentle penetrating warmth. In birds' 

 eggs, for instance, incubation causes this rise of temperature, and in 

 many other eggs a gentle warmth of the atmosphere is enough ; lastly, 

 circumstances that favour germination vitalise the seeds of plants. 



But eggs and seeds adapted for giving existence to animals and 

 plants must of necessity contain, a fertilised embryo enclosed in in- 

 vestments, whence it can only emerge after breaking through them : 

 such eggs and seeds are therefore products of sexual reproduction, 

 since reproductive bodies otherwise originating do not have any 

 embryo enclosed in investments which have to be broken through at 

 the outset of development. Gemmae and the more or less oviform 

 reproductive bodies of many animals and plants cannot assuredly 

 be compared with them : it would be a waste of time to search for 

 sexual generation where nature has had no means for establishing it. 



