210 



THE INDUCTIONS OF BIOLOGY. 



the same form. These two distinct processes of multiplication, 

 may be aptly termed homogenesis and heterogenesis* Under 

 these heads let us consider them more closely. 



The kind of genesis, once supposed to be universal, in 

 which the successive generations are alike, is always sexual 

 genesis ; or, as it has been otherwise called gamogenesis. In 

 every species of organism which multiplies by homogenesis, 

 each generation consists of males and females ; and from the 

 fertilized germs they produce, the next generation of similar 

 males and females arises. This method of propagation ia 

 further distinguished by the peculiarity, that each fertilized 

 germ gives rise to but one individual the product of de- 

 velopment is always organized round one axis, and not round 

 several axes. .Between the different kinds of homo- 



genesis, the most marked contrast, and the only one which 

 need here detain us, is that between the oviparous and the 

 viviparous. The oviparous kind is that in which the fertil- 

 ized germ is detached from the parent, before it has 

 undergone any considerable development. The viviparous 

 kind is that in which development is considerably advanced, 

 or almost completed, before final detachment takes place. 

 This distinction is, however, not a sharply-defined one : there 

 are transitions between the oviparous and the viviparous 

 processes. In ovo-viviparous genesis, there is an internal 

 incubation ; and though the young are in this case finally 

 detached from the parent in the shape of eggs, they do not 

 leave the parent's body until after they have assumed 

 something like the parental form. Looking 



around, we find that hornogenesis is universal among the 

 Vertebrata : there is no known vertebrate animal but what 

 arises from a fertilized germ, and unites into its single indi= 

 viduality the whole products of this fertilized germ. In 



* Unfortunately the word twtercgenesis, has hecn already used as a synoiiymf- 

 for " spontaneous generation." Save by those few who believe in " spontaneous 

 generation," however, little objection will be felt to using the word iii a sense 

 that seems much more appropriate. 



