154 Darwin, and after Darwin. 
Vertebrata, the heart is constructed upon the fish-like 
‘type. That is to say, it is placed 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 
