. 43 



future sex, and that at every period of life, there 

 is not, in either, any organ which is not obvious- 

 ly repeated in the other however apparently 

 useless it may be in one of them as if nature, 

 in conformity to a certain law, could not avoid 

 constructing both after the same model. And, 

 with respect to the analogies between certain 

 organs of the human body and the correspond- 

 ing orgaiis of the inferior tribes of animals, these 

 are so obvious, when we compare, for example, 

 the arm of a- man with the fore-leg of a quadru- 

 ped, the wing of a bird, the anterior extremity 

 of a reptile, or the thoracic fin of a fish in all 

 their parts, soft as well as hard that nobody 

 can refuse to assent to them. So far, a degree 

 of unity of organization is, and must be, uni- 

 versally admitted ; and, if we take almost any 

 other organ of man and the superior animals, it 

 would not be difficult to trace its prototype in 

 some corresponding organ in the very lowest 

 tribes. Thus man and quadrupeds birds and 

 reptiles have lungs but no gills ; while fishes, 

 on the contrary, have gills but no lungs : but all 

 the former had, at an early period of their de- 

 velopement, gills and no lungs ; and the lungs, 

 which even man at length acquires, are easily 

 traced through the fleshy lungs of birds, and 

 the membranous lungs of reptiles of serpents 

 in particular to the air-bladder of fishes, as 

 their prototype. Again, the mammalia and 

 birds have a double heart, and but few large 

 blood vessels immediately connected with it ; 



