SPHERE AND FUNDAMENTAL 



groups, according to their mode of respiration ; uniting iu 

 one group all those which breathe by gills, and, in the 

 other, those which breathe by lungs ; but this distinction loses 

 its importance, when we consider that various animals, as, for 

 example, frogs, which respire by lungs in the adult state, 

 have only gills when young : hence it is evident that the 

 respiratory organs cannot be taken as a satisfactory basis 

 for fundamental classification. They are, as we shall see, 

 subordinate to a more important organism, namely, the ner- 

 vous system. 



20. Again, we ave a means of appreciating the relative 

 grade of animals by the comparative study of their develop- 

 ment. It is evident that the caterpillar, in becoming a butter- 

 fly, passes from a lower to a higher state ; clearly, therefore, 

 animals resembling the caterpillar, as, for instance, worms, 

 occupy a lower rank than insects. There is no animal which 

 does not undergo a series of changes similar to those of the 

 caterpillar or the chicken ; only, in many of them, the most 

 important ones occur before birth, during what is called the 

 embryonic period. 



21. The life of the chicken has not just commenced when 

 it issues from the egg ; for, if we break the shell some days 

 previous to the time of hatching, we find in it a living animal, 

 which, although imperfect, is nevertheless a chicken ; it has 

 been developed from a hen's egg, and we know that, should it 

 continue to live, it will infallibly display all the character- 

 istics of the parent bird. Now, if there existed in nature an 

 adult bird, as imperfectly organized as the chicken on the day 

 before it was hatched, we should assign to it an inferior rank. 



22. In studying the embryonic states of the mollusks or 

 worms, we observe in them points of resemblance to many 

 animals of a lower grade, to which they afterwards become 

 entirely dissimilar; for example, the myriads of minute aquatic 

 animals embraced under the name of Infusoria, whose organ- 

 ization is generally very simple, remind us of the embryonic 

 forms of other animals. We shall have occasion to show that 

 the Infusoria are not to be considered as a distinct class of 

 animals, but that among them, there are found members of all 

 the lower classes of animals, as mollusks, crustaceans, polyps, 

 and even vegetable organisms. 3 



3 And a;' grouped in the families Desmidice and Diatomacece. ED. 



