WHAT IS A SPECIES? 15I 



universally admitted by all naturalists until Darwin came 

 to show that this category was also variable, changing, 

 and without fixity. The partisans of the first theory ac- 

 corded to the species a sum of particular immutable char- 

 acters; it had always been such as we see it, {species tot 

 sunt divcrsa; qiiot diver sec forma; sunt creates). . . . The parti- 

 sans of the theory of Transmutation believe that species 

 have appeared slowly, the one after the other, and by suc- 

 cessive transformations. ... In order to limit living species, 

 the better criterion is furnished by their direct descent. 

 According to Cuvier, one should refer to the same species all 

 the individuals which were born, the one from the other, or 

 of common parents, and which resemble each other as much 

 as they resemble their parents; the individuals of separate 

 species are incapable of fertile union, or produce generally 

 only infertile progeny. In paleontology it is impossible to 

 control real consanguinity by physiological observation, and 

 consequently we are deprived of this criterion in the study 

 of fossil species. . . . One ought to recognize, moreover, that 

 the value of this criterion is not more absolute in the deter- 

 mination of living botanical or zoological species, as numerous 

 species are capable of reproduction without sexual union, (as 

 hermaphrodites, the products of scissiparity, budding, alternate 

 generation, parthenogenesis), and there are other species, 

 recognized as good species, the crossing of which produces 

 fertile hybrids. ... If, then, the delineation of species is dififi- 

 cult in botany and zoology, it is evident that it will be more 

 so in paleontology. The paleontologist is limited to a knowl- 

 edge of the exterior forms of fossils, and these, moreover, 

 often incomplete, the better characters having been frequently 

 destroyed by fossilization. ... In general, there are referred 

 in paleontology to the same species a// the individuals, or all 

 the fragnients, which present certain coninioii characters, and 

 form a circumscribed gro7ip, independent of geological range or 

 geographical distrihition ; they can, nevertheless, be associated 

 zvitJi neighboring groups by a small number of intermediate 

 forms. 



The Theory of Mutability of Species and Evolution. — Bonnet 

 (1720— 1793) advanced the idea that diversity of climate, nour- 



