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 occur 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 l . 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. KIELMEYEB, Ueber die Verhdltnisse der organischen Krdfte unter einander 

 in der Reihe der verschiedenen Organisationen. Tubingen, 1814. Svo. s. 38. The differ- 

 ent works of CAEUS, TIEDEMANN and J. F. MECKEL supply many examples of the 

 application of this position. 



