UNITY OF DESIGN. 443 



go a series of changes precisely analogous to those of the 

 tadpole. 



Mammalia, during the early periods of their development, 

 are subjected to all the transformations which have been now 

 described, commencing 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 development 

 of various parts of the system, more especially of the brain, 

 the ear, the mouth, and the extremities, is carried still far- 

 ther 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 con- 

 sisting 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, with a 

 caudal prolongation of the sacrum, and an intermaxillary 

 bone; and, lastly, changing its structure to one adapted to 

 the erect position, accompanied by a great expansion of the 

 cerebral hemispheres, which extend backwards so as com- 

 pletely to cover the cerebellum. Thus does the whole fab- 

 ric arrive, by a gradual process of mutation, at an extent of 

 elaboration and refinement, unattained by any other race of 

 terrestrial beings, and which has been justly regarded as 

 constituting the climax of organic developmentt 



* 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. 



t A popular opinion has long prevailed, even among the well informed, 

 that misshapen or monstrous productions, or Iitsus naturae, as they were 

 termed, exhibit but the freaks of nature, who was believed, on these occa- 

 sions, capriciously to abandon her usual course, and to amuse herself in the 

 production of grotesque beings, without any special object. But it is now 

 found that all defective formations of this kind are occasioned by the imper- 



