METAMOKPIIOSES OF AOTMALS. 361 



mammals, at birth, are destitute of teeth, and incapable of 

 r limbs ; ami all are dependent on the mother and 

 the milk secreted by her, until the stomach is capable of 

 ini* other aliment. 



'. If it be thus shown that the transformations which 

 take place in the embryo are of the same nature and of the 

 importance as those which occur afterwards, the cir- 

 cumstance that some precede and others succeed birth, cannot 

 mark any radical distinction between them. Both are pro- 

 cesses of the life of the individual. Now, as life does not 

 commence at birth, but goes still farther back, it is quite clear 

 that the modifications w r hich supervene during the former 

 period are essentially the same as the later ones ; and hence 

 that metamorphoses, far from being exceptional in the case 

 of insects, are one of the general features of the animal king- 

 dom. 



. We are therefore perfectly entitled to say that all 

 animals, without exception, undergo metamorphoses. Were 

 it not so, we should be at a loss to conceive why animals of 

 the same division present such wide differences ; and that 

 there should be, as in the class of reptiles, some families that 

 undergo metamorphoses (.the frogs, for example), and others 

 in which nothing of the kind is observed after birth (the 

 lizards and tortoises). 



575. It is only \)j connecting the two kinds of trans- 

 formation namely, those which take place before, and those 

 after birth, that we are furnished with the means of ascer- 

 taining the relative perfection of an animal ; in other words, 

 these transformations become, under such circumstances, a 

 natural key to the gradation of types. At the same time, 

 they force upon us the conviction that there is an immu- 

 table principle presiding over all these changes, and regulat- 

 ing them in a peculiar manner in each animal. 



576. These considerations are important, not only from 

 their bearing on classification, but not less so from the appli- 

 cation which may be made of them to the study of fossils. 

 If we examine attentively the fishes that have been found in 

 the different strata of the earth, we remark that those of the 

 most ancient deposits have in general preserved only the 

 apophyses of their vertebrae, whilst the vertebrae themselves 

 are wanting. Were the sturgeons to become petrified, they 



