DEVELOPMENT OF ANIMALS. 25 
surface appears to discharge the functions of absorption and nu- 
trition. 
A gradual course of development, similar to that observed in 
the animal series, is also pursued by the embryo of the more 
perfect animals. The whole Life is Metamorphosis; and there are 
animals in which the change of form is so great and so remarkable, 
that it does not escape even the eye of the multitude. Thus, for 
instance, a Caterpillar is changed into a Butterfly: a creeping, dull, 
voracious creature into one that flies and runs, and scarcely takes 
any food. In the same way the metamorphoses of Frogs are 
notorious. But there are other animals whose metamorphoses do 
not oecur in so striking a manner, but are principally limited to the 
earliest periods of life. Every animal is slowly developed, and 
becomes more perfect as new organs are added to those already 
present. But this idea must not be so apprehended as if a Mam- 
mal, for instance, had been at first an Infusory, then a Polyp, a 
Medusa, afterwards an Insect, a Fish, a Bird, &c., as some express 
themselves! This would be as extravagant as it is unfounded: 
but properly, as we conceive, many moderns assume that all the 
organs in different periods of life do pass through a development 
and metamorphosis, and that the structure of a perfect animal, in 
its foetal state, is more simple, and corresponds with that of the 
lower animals of the same Type to which itself belongs. Thus the 
first rudiments of all vertebrate animals are similar, and the history 
of the development of the Chick may illustrate that of Mammals in 
the first periods. This is more than a phrase without proof: rather 
is it the result of very numerous observations—for instance, those 
on the Brain and the Heart in the human embryo—and we shall 
find it confirmed by frequent instances in the course of the present 
work. 
We have already on various occasions made use of the terms 
‘imperfect’ and ‘perfect’ animals, and shall have to use them 
often. But since every animal is perfect in its kind, the term 
1 How this gradual progress of the embryo through the different gradations of the 
animal kingdom is to be understood cannot here be further particularized. Compare 
hereon C, F. KigetmryeEr, Ueber die Verhdiltnisse der organischen Kréfte unter einander 
in der Rethe der verschiedenen Organisationen. Tubingen, 1814. 8vo. s. 38. The differ- 
ent works of CARUS, TIEDEMANN and J. F. MEcKEL supply many examples of the 
application of this position. 
