352 EEPRODUCTICXN". 



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

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

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

 favourable conditions, may be hatched. It is probable that 

 each animal affords the proper conditions for some particular 

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

 animals have parasites peculiar to themselves. 



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

 them, the Rotifer a especially, lay eggs. These eggs, which 

 are extremely minute (some of them only 1-1 2,000th of an 

 inch in diameter), are scattered everywhere in great profusion, 

 in water, in the air, in mist, and even in snow. Assiduous 

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

 have followed their development, and have seen the young 

 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 sixth 

 generation. 



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

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

 assign to them a spontaneous origin altogether incompatible 

 with what we know of organic development. Their rapid 

 appearance is not at all astonishing, when we reflect that 

 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 any 

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



* In this connection it ought to be remembered that a large proportion 

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

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

 many must be referred to the vegetable kingdom. 



