634 UNITY OF DESIGN. 



with reference to a branchial circulation. In 

 their conversion to the purposes of aerial respi- 

 ration, they undergo a series of changes pre- 

 cisely analogous to those of the tadpole. 



Mammalia, during the early periods of their 

 dev elopement, are subjected to all the transform- 

 ations which have been now described ; com- 

 mencing with an organization corresponding to 

 that of the aquatic tribes ; exhibiting not only 

 branchiae, supported on branchial arches, but 

 also branchial apertures in the neck ; and thence 

 passing quickly to the conditions of structure 

 adapted to a terrestrial existence. The deve- 

 lopement of various parts of the system, more 

 especially of the brain, the ear, the mouth, and 

 the extremities, is carried still farther than in 

 birds. Nor is the human embryo exempt from 

 the same metamorphoses ; possessing at one 

 period branchiae and branchial apertures similar 

 to those of the cartilaginous fishes,* a heart with 

 a single set of cavities, and a brain consisting of 

 a longitudinal series of tubercles ; next losing 

 its branchiae, and acquiring lungs, while the 

 circulation is yet single, and thus imitating the 

 condition of the reptile ; then acquiring a double 

 circulation, but an incomplete diaphragm, like 

 birds ; afterwards, appearing like a quadruped, 



* These facts are given on the authorities of Rathke, Baer, 

 Huschke, Breschet, &c. Ann. des Sc. Naturelles, xv. 266. See 

 also the paper of Dr. A. Thomson, already quoted. 



