212 THE DOCTRINE OF DESCENT. 



doubt that this is an instance of adaptation to unusual 

 conditions. If the mode of life of the marsupial frog, 

 which carries its young in a membranous fold of the 

 back, and the Surinam toad, of which the larvae live 

 singly in the chambers of a kind of honeycomb on the 

 back, were better known than they are, we should 

 assuredly arrive at the same results as with the black 

 salamander. In the absence of ether knowledge, the 

 observations of M. Bavey, Marine Pharmaceutist at 

 Guadaloupe, first published in 1873, are of the highest 

 importance. 66 A frog of those parts (Hylodon Martini- 

 censis) goes through its whole metamorphosis in the 

 egg. In the egg it has gills and tail ; and from the brief 

 remark that the island contains only rapid running 

 streams, and nowhere stagnant waters or marshes, it 

 appears that this is also a case in which adaptation 

 modifies and curtails development. 



If, after this introduction, we now examine the so- 

 called direct development with more attention, it may 

 in every way be compared to the metamorphosis of 

 the Hylodes of Guadaloupe. Direct development is 

 a transformation in the ovum ; and in the cases in 

 which it occurs, the phases of embryonic development 

 are repetitions, more or less distinct, of the historic 

 development of the family. We will only particularize 

 in the embryonic life of the Vertebrata (in which 

 metamorphosis does not take place), some phases that 

 are stages of curtailed transformation, and recapitu- 

 late the permanent condition of their progenitors. It 

 has been repeatedly , mentioned that in all vertebrate 

 animals, the vertebral column is first laid out as an 

 unsegmented cord and an unsegmented sheath for the 



