652 ZOOLOGY. 



sen, afterward, as a pnpa, becoming fixed to a rock ; the 

 young oyster-spat swims freely about, finally becoming fixed 

 to the bottom. This change of life and of form undoubted- 

 ly tends to prevent the extinction of the species, since, if at 

 a given moment the parents were swept out of existence, 

 the young living in a different station would continue to 

 represent the species. This law is seen to hold good among 

 insects, where many species are represented in- the winter- 

 time by the egg alone, others by the caterpillars, others by the 

 chrysalis, while still others hybernate as imagines. Again, 

 in the marine species, the free-swimming young are borne 

 about by ocean and tidal currents, and in this way what in 

 adult life are the most sedentary forms become widely dis- 

 tributed from coast to coast and sea to sea. On the other 

 hand, the larval forms of fixed marine animals serve as food 

 for fishes, especially young fishes and numerous inverte- 

 brates, while their stationary parents afford subsistence for 

 still other forms of life ; thus were it not for the metamor- 

 phoses of animals, many species would become extinct 

 sooner than they do, while the great overplus of larval 

 forms gives to many other species of animals a hold on ex- 

 istence. 



Metamorphosis among the invertebrate animals, espe- 

 cially, is j)erhaps the rule and not the exception. Where ani- 

 mals develop directly, as in certain insects, crustaceans, cer- 

 tain salamanders, toads and frogs, this is due to some 

 change in the environment ; in the case of Amphibians, 

 perhaps the want of water, or some other cause, there always 

 being an adaptation in the case of the direct mode of de- 

 velopment to the surroundings of the animal and the require- 

 ments of its existence. 



Parthenogenesis, and Alternation of Generations. — 

 Having traced the normal jDrocess of develoj)ment of ani- 

 mals, we may turn to certain unusual or abnormal modes of 

 production. As an example of what is known as " alternation 

 of generations," may be cited the mode of development of the 

 jelly-fish, such as the naked-eyed medusas [Melicertum and 

 Campatmlaria), which at one time of life develop by budding, 

 at another by eggs ; of the trematode worms, the adult forms 



