C. E. Beecher — Origin and Significance of Spines. 5 



considered from the concrete, objective side, the limits 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 divisions given, in 

 itself furnishes sufficient evidence that these have been the 

 limits of organic change, at least under present terrestrial con- 

 ditions. This does not imply that the phylogenies 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 varia- 

 tion in each type of epidermal structure. In vertebrate ani- 

 mals 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 grada- 

 tions 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. 



Definition of Terms. 



In the beginning, it is well to understand the meaning and 

 extent of the terms included under the comprehensive word 

 spine. In a general sense, spine is here used to cover any stiff, 

 sharp-pointed process. A prickle is restricted in use to the 

 small, sharp-pointed, conical projections which are purely 

 cuticular ; as in the Rose and Blackberry. A thorn is a sharp 

 process on plants, usually representing a branch or stem. A 

 horn is an excrescence on the head of certain animals, and is 

 properly hollow. An antler is a solid bony process, usually 



