612 THE REPRODUCTIVE FUNCTIONS. 



emerge from the water when they have acquired 

 wings ; and also in the steps of transition from 

 the tadpole to the frog. But similar, though less 

 conspicuous changes occur in the higher verte- 

 brated animals, during the early periods of their 

 formation, corresponding to the differences in 

 the modes of aeration employed at different 

 stages of developement. In the primeval con- 

 ditions this function is always analogous to that 

 of aquatic animals, and requires for its perform- 

 ance only the simpler form of heart already 

 described, consisting of a single set of cavities ; 

 but the system being ultimately designed to 

 exercise atmospheric respiration, requires to be 

 gradually adapted to this altered condition ; and 

 the heart of the Bird and the Quadruped must 

 be separated into two compartments, corre- 

 sponding to the double function it will have 

 to perform. For this purpose a partition wall 

 is built in its cavity ; and this wall is begun 

 around the interior circumference of the ven- 

 tricle, and is gradually carried on towards the 

 centre; there being, for a time, an aperture of 

 communication between the right and left cavi- 

 ties; but this aperture is soon closed, and the 

 ventricle is now effectually divided into two. 

 Next the auricle, which at first was single, 

 becomes double ; not, however, by the growth of 

 a partition, but by the folding in of its sides, 

 along a middle line, as if it were encompassed 



