8 STUDIES IN EVOLUTION 



between which are found all the variations actually presented 

 by any character or set of characters, in the animal or the 

 vegetable kingdoms, can at once be determined. The fact 

 that the organic world can be divided into kingdoms, sub- 

 kingdoms, classes, orders, etc., and definitions of the divi- 

 sions given, in itself furnishes sufficient evidence that these 

 have been the limits of organic change, at least under present 

 terrestrial conditions. This does not imply that the phylog- 

 enies of groups of animals and plants do not converge and 

 coalesce, and join larger and larger phyla in past ages, so 

 that the gaps between unlike forms are gradually filled by 

 complete series. It does, however, express the definite 

 heterogeneity of the results of development. 



For the sake of illustrating an extreme range of variation, 

 it will be granted that the assemblage of characters by which 

 a mammal is now recognized precludes mammalian variation 

 into a cold-blooded, non-vertebrate, lungless animal. Like- 

 wise the mammalian skeleton cannot be siliceous or chitinous. 

 Externally mammals may be smooth, hairy, scaly, or plated, 

 but not feathered. There may be found numerous gradations 

 from the smooth to the plated state, and a great range of 

 variation in each type of epidermal structure. In vertebrate 

 animals generally, the hair may vary in length, in fineness, 

 in color and shape ; it may form bristles, or spines, or feathers ; 

 and, as a skin character, it is related to horn-sheaths, hoofs, 

 nails, claws, scales, and teeth. These constitute the limits 

 of modification in epidermal or exoskeletal growths. The 

 types are few, but the variety in each is almost infinite. 

 The variation may be seen in individuals, but becomes greater 

 in species, and increases still more in larger groups. The 

 gradations are numerous between the hair of a Beaver and 

 the spines of a Porcupine; between the horns of the Giraffe, 

 Rhinoceros, and Antelope ; between the nails of Man and the 

 claws of the Carnivora ; and between the teeth of a Dog-fish 

 and those of a Tiger. 



