352 REPRODUCTION. 



545. All animals swallow, in the same manner, with thei 

 food, and in the water they drink, numerous eggs of such pa 

 rasites, any one of which, finding in the intestine of the anima 

 favourable conditions, may be hatched. It is probable tha 

 each animal affords the proper conditions for some particula 

 species of worm ; and thus we may explain how it is that mos 

 animals have parasites peculiar to themselves. 



546. As respects the infusoria, we also know that most o 

 them, the Rotifera especially, lay eggs. These eggs, whicl 

 are extremely minute (some of them only l-12,OOUth of ai 

 inch in diameter), are scattered everywhere in great profusion 

 in water, in the air, in mist, and even in snow. Assiduou: 

 observers have not only seen the eggs laid, but, moreover 

 have followed their development, and have seen the youn< 

 animal forming in the egg, then escaping from it, increasing 

 in size, and, in its turn, laying eggs. They have been able 

 in some instances, to follow them even to the fifth and sixtl 

 generation. 



547. This being the case, it is much more natural to sup 

 pose that, the infusoria* are products of like germs, than t( 

 assign to them a spontaneous origin altogether incompatibL 

 with what we know of organic development. Their rapic 

 appearance is not at all astonishing, when we reflect tha 

 some mushrooms attain a considerable size in a few hours 

 but yet pass through all the phases of regular growth ; and 

 indeed, since we have ascertained the different modes of gene 

 ration among the lower animals, no substantial difficulties anj 

 longer exist to the axiom " omne vivum ex ova" ( 433). 



* In this connection it ought to he remembered that a large proportior 

 of the so-called Infusoria are not independent animals, but immature 

 germs, belonging to different classes of the animal kingdom, and thai 

 many must be referred to the vegetable kingdom. 



