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higher vertebrata, are passed through previous to birth; but in 
many of the lower animals, even in some of the vertebrated, the 
changes of form are not completed till after birth. We are all. 
familiar with the tadpoles, in our ditches in spring, which turn 
to frogs, and the caterpillars, on trees and various plants, which 
turn to butterflies and moths. But further; many of these 
primitive forms of animal life exist, and are to be met with 
almost everywhere, which never rise to any higher form of 
structure. Thus, there are worms and centipedes and slugs, the 
two former often resembling the larval states of some insects, 
which never become anything else. They remain worms and 
centipedes and slugs as long as they live. 
There are also certain cases in which the metamorphosis above 
spoken of, owing to the conditions of the environment, is held 
back and never completed ;—or it is completed with the omission 
of one stage of it ;—or it is completed prematurely ;—or by a law 
of the species it is confined, in its full development, to one sex 
only. To give examples of such irregularities :— 
If tadpoles are kept in confinement, with the light entirely 
excluded, it has been said that further development is arrested, and 
that they rarely turn to frogs. If frogs and toads get incarcerated 
in cellars or other places, where they can have no access to water 
at the time of the breeding season, the tadpole state is passed 
over altogether, and the young when hatched come forth fully 
formed at once.* 
The same thing takes place in the newt (Zriton), which is 
ordinarily born in the water, and completes its metamorphosis 
in that element ; but young newts may often be found on land, 
under stones or in damp buildings and other places, from one to 
four inches in length and in the perfect state. These have either 
been born on land, and the tadpole state passed through while 
* See Ann. and Mag. Nat, Hist., 2nd sev., vol, xi., p. 341, and the 
same vol. p. 482. 
