652 ZOOLOGY, 
sea, afterward, as a pupa, 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 undoukted- 
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 perhaps the rule and not the exception. Where aii- 
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 process of development of ani- 
mals, we may turn to certain unusual or abnormal modes of 
production. Asan 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 medusee (Melicertum and 
Campanularia), which at one time of life develop by budding, 
atanother by eggs ; of the trematode worms, the adult forms 
