There are abundant evidences of the chemical differentiation of species, which in some instances is as prominent as the morphologic distinctions. For example, we find specific varieties of hemoglobin, which, although functionating similarly in all animals, and with closely related composition, yet show quite different crystalline forms. Probably the differences in the effects of drugs on different animals depend on different chemical composition of certain parts of the animals. The entire work on immunity testifies to these chemical peculiarities of divisions in the animal kingdom. Even natural selection plays a part in such differentiation, as seen in the protective colorings, the venoms, the defensive odor of the skunk, or the ink discharged by the cuttle-fish. The peculiarity of proteid metabolism by which in reptiles and birds the chief end product is uric acid, while in mammals it is urea, has been the pivotal point for most of the study, theorization
CHEMICAL INDIVIDUALITY.. JAMA. 1903;XL(17):1146. doi:10.1001/jama.1903.02490170030003
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