230 SYSTEM OF LIVING FORMS. 



to produce animals as widely different as they are in the full- 

 grown state, simply by successive, gradual metamorphoses ; 

 and these metamorphoses upon one and the same plan, accord- 

 ing to one and the same general progress." Again : " It may 

 therefore be said, with perfect propriety, that the higher Ver- 

 tebrates undergo changes, through which, in different periods 

 of their life, they resemble the lower ones ; that there is a 

 period when the young bird has not only the form, but the 

 structure, and even the fins, which characterize the Fish. And 

 of the young Mammals the same may be said. There is a 

 period in the structure of the young Rabbit (in which the in- 

 vestigations have been traced more extensively than in other 

 species), when the young Rabbit resembles so closely the 

 Fish, that it even has gills, living in a sac full of water, breath- 

 ing as Fishes da So that the resemblance is as complete as 

 it can be, though each of these types grows to a complication 

 of structure, by which the young Mammal, for instance, leav- 

 ing behind this low organization of the lower types, rises to a 

 complication of structure, to higher and higher degrees, and to 

 that eminence even which characterizes mankind."* 



These facts certainly show a unity of plan and a progressive 

 succession, of, in some sense, mutually dependent forms, in the 

 system of animated nature, which countenances the idea that 

 the whole creation of lower animals is, as it were, the foetus 

 of the whole human creation, and that the latter was thus de- 

 veloped by a process somewhat resembling that which the 

 author of the " Vestiges of Creation" supposed to have taken 

 place, and which he calls " the universal gestation of Nature ;" 

 yet we shall soon see that, so far from this theory dispensing 

 with the agency of a God, this universal gestative process 

 could not have proceeded even through its first stages, without 

 * Lectures on Comparative Embryology, Lee. xiL 



