154 Darwin, and after Darwin. 



Vertebrata, the heart is constructed upon the fish-hke 

 type. That is to say, it is phiced far forwards, and, 

 from having been a simple tube as in Worms, is now 

 divided into two chambers, as in Fish. Later on it 

 becomes progressively pushed further back between 

 the developing lungs, while it progressively acquires 

 the three cavities distinctive of Amphibia, and finally 

 the four cavities belonging only to the complete 

 double circulation of Birds and Mammals. Moreover, 

 it has now been satisfactorily shown that the lungs 

 of air-breathing Vertebrata, which are thus destined 

 to supersede the function of gills, are themselves the 

 modified swim-bladder or float, which belongs to Fish. 

 Consequently, all these progressive modifications in 

 the important organs of circulation and respiration in 

 the air-breathing Vertebrata, together make up as 

 complete a history of their aquatic pedigree as it 

 would be possible for the most exacting critic to 

 require. 



If space permitted, it would be easy to present 

 abundance of additional evidence to the same effect 

 from the development of the skeleton, the skull, the 

 brain, the sense-organs, and, in short, of every con- 

 stituent part of the vertebrate organization. Even 

 without any anatomical dissection, the similarity of 

 all vertebrated embryos at comparable stages of de- 

 velopment admits of being strikingly shown, if we 

 merely place the embryos one beside the other. 

 Here, for instance, are the embryos of a fish, a sala- 

 mander, a tortoise, a bird, and four different mammals. 

 In each case three comparable stages of development 

 are represented. Now, if we read the series horizontally 

 we can see that there is very little difference between 



