AND THE LOWER ANIMALS. 279 



"beings whose forms and modes of life are quite dis- 

 similar,, if we desire to know the species. Had not 

 experience informed us, who could have suspected the 

 existence of the Distoma under its forms of ciliated 

 larva, Sporocyst, and of free and encysted Cercariae ? 



Metamorphosis, under the form of geneagenesis, not 

 only complicates the idea which the mind conceives 

 of any particular species, but it even extensively 

 modifies our general and abstract notions of species. 

 Up to this, we have understood by this word a succes- 

 sion of beings proceeding one from the other, and 

 whose individuality is maintained despite a number 

 of minor less apparent changes. At present, we must 

 add to this, that, in certain cases, the species is com- 

 posed of perfectly distinct beings, which proceed one 

 from another by a process of multiplication. To the 

 idea of continuity of individuals, which forms the 

 basis of all existing definitions, we must connect that 

 of succession of cycles. This is what was first under- 

 stood by Chamisso, and was fully demonstrated by 

 Steenstrup. 



The general phenomenon which we are now con- 

 sidering seemed for a long while, under its ' forms 

 of true metamorphosis and geneagenesis, to supply an 

 argument to the advocates of spontaneous generation. 

 Until the time of Eedi and Vallisnieri, insect larvae 

 were thought to be formed by the action of the 

 physico-chemical forces on decomposing organic 

 matter. Even in a few modern works it is asserted 

 that the intestinal worms are the immediate products 

 of the animal in which they are found. We have 

 seen that the best-ascertained facts lead to a diame- 

 trically opposite conclusion. For a long period it has 



