GENESIS. 271 



contrast with heterogenesis as exhibited in species which dis- 

 play distinct sexuality, has also the characteristic that each 

 new individual begins as an egg detached from the maternal 

 tissues, instead of being a portion of protoplasm continuous 

 with them, and that its development proceeds independently. 

 This development may be carried on either internally or ex- 

 ternally; whence results the division into the oviparous and 

 the viviparous. The oviparous kind is that in which the 

 fertilized germ is extruded 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 extrusion takes place. This 

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

 transitions between the oviparous and the viviparous pro- 

 cesses. In ovo-viviparous genesis there is an internal incuba- 

 tion ; and though the young are in this case finally extruded 

 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 



homogenesis is universal among the Vertebrata. Every 

 vertebrate animal arises from a fertilized germ, and unites 

 into its single individuality the whole product of this 

 fertilized germ. In the mammals or highest Vertebrata, this 

 homogenesis is in every case viviparous; in birds it is uni- 

 formly oviparous; and in reptiles and fishes it is always 

 essentially oviparous, though there are cases of the kind 

 above referred to, in which viviparity is simulated. Passing 

 to the Invertebrata, we find oviparous homogenesis universal 

 among the Arachnida (except the Scorpions, which are ovo- 

 viviparous) ; universal among the higher Crustacea, but not 

 among the lower; extremely general, though not universal, 

 among Insects; and universal among the higher Mollusca 

 though not among the lower. Along with extreme inferiority 

 among animals, we find homogenesis to be the exception 

 rather than the rule; and in the vegetal kingdom there 

 appear to be no cases, except among the Algce and a few 



