PRINCIPLES OF ZOOLOGY. 31 



18. Our investigations should not be limited to adult 

 animals, but should also include the changes which they 

 undergo during the whole course of their development. 

 Otherwise, we shall be liable to exaggerate the importance 

 of certain peculiarities of structure which have a predomi- 

 nant character in the full-grown animal, but which are shaded 

 off, and vanish, as we revert to the earlier periods of life. 



19. Thus, for example, by regarding only adult individu- 

 als, we might be induced to divide all animals into two 

 groups, according to their mode of respiration ; uniting, on 

 the one hand, all those which breathe by gills, and, on the 

 other, those which breathe by lungs. But this distinction 

 loses its importance, when we consider that various animals, 

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

 state, have only gills when young. It is thence evident that 

 the respiratory organs cannot be taken as a satisfactory 

 basis of our fundamental classification. They are, as we 

 shall see, subordinate to a more important system, namely, 

 the nervous system. 



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

 grade of animals by the comparative study of their devel- 

 opment. It is evident that the caterpillar, in becoming a 

 butterfly, passes from a lower to a higher state. Clearly, 

 therefore, animals resembling the caterpillar, the worms, for 

 instance, must occupy a lower rank than those approaching 

 the butterfly, like most 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 egg some days 

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

 mal, which, although imperfect, is nevertheless a chicken : 



