REPRODUCTION 



forms — often so different from each other that they were 

 formerly thought to be animals of separate classes, such 

 as the polyps and medusae. Hence we must not describe 

 the reproductive circle of the phanerogams as an al- 

 ternation of generations, although it has started from 

 the fern (by abbreviated heredity). 



All simple forms of sexual reproduction without alter- 

 nation of generations are comprised under the title of 

 hypogenesis. The generative cycle proceeds from ovum 

 to ovum in one and the same bion or physiological 

 individual. This form of development is usual with 

 most of the higher animals and plants; it may proceed 

 with or without metamorphosis. The younger forms 

 which arise temporarily in the latter case, and are distin- 

 guished from the sexually ripe form by the possession of 

 the provisional (and subsequently disappearing) organs 

 — larva organs (for instance, the tadpole or the pupa), 

 are comprised under the general head of larvas. 



As a rule, only organisms of the same species seem 

 to have sexual union and generate fertile progeny. This 

 was formerly a rigid dogma, and served the purpose of 

 defining the loose idea of the species. It was said: 

 "When two animals or plants can have fertile offspring 

 they belong to the same real species." This principle, 

 which once afforded support to the dogma of the con- 

 stancy of species, has long been discarded. We now 

 know by numbers of sound experiments that not only 

 two closely related species, but even two species of 

 different genera, may have sexual intercourse in certain 

 circumstances, and that the hybrids thus generated can 

 have fertile offspring, either by union among themselves 

 or with one of the parents. However, the disposition to 

 hybridism varies considerably, and depends on the un- 

 known laws of sexual affinity. This sexual affinity must 

 be based on the chemical properties of the plasm of 

 the copulating cells, but it seems to show a good deal of 



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