ANIMALS AS MODIFIED BY ENVIRONMENT. 245 



these. This process is repeated again and again until groups of 

 animals are reached in which the differences of structures sepa- 

 rating them from others become so minute that they are called 

 varietal or specific^ while the differences of environment to which 

 these subdivisions are adapted grow fainter and fainter, until, 

 when the point is reached where the classifier is compelled to 

 throw together a number of animals and call them a species, be- 

 cause he can no longer find structural differences to form di- 

 visions upon, there the student of animal habits will find the 

 same animals related to practically identical facts of environ- 

 ment. 



An example of such changes in environment and of such fol- 

 lowing modifications of structures may be taken from the mam- 

 mals. Primarily arboreal in habits, and climbing, with hands 

 developed on one or both pairs of limbs, their skulls and jaws 

 and teeth were fitted for masticating their food. All of their 

 sujDposed ancestry had simply swallowed their food whole. 



This mammalian character of the teeth, when attained to, 

 made of importance, for the first time, such differences of foods 

 as necessitated different kinds of teeth for masticating them. 

 Examined in this respect, the food ordinarily made use of by 

 mammals may be roughly divided into five classes — fruits, in- 

 sects, flesh, grass, and hard foods — and these have led to the for- 

 mation of the five great orders: primates, insectivores, carni- 

 vores, ungulates, and rodents. The importance of these food 

 characteristics in modifying mammals may be seen when it is 

 stated that these five orders, with the bats, which are flying in- 

 sectivores, contain five sixths of the mammalian species and 

 probably more than nineteen twentieths of all the individuals of 

 the class. Again, the becoming fitted for hard foods, by the 

 rodents, brought them under the influence of a new set of sur- 

 roundings, namely, the various locations in which such foods 

 existed, and the families of squirrels, mice, hares, beavers, etc., 

 are the result. The same process again took place in the forma- 

 tion of the genera and species of these families. 



We must look upon the class, order, family, genus, and spe- 

 cies characters of each individual animal as structures which fit 

 it or have fitted its ancestors for as many distinct facts of en- 

 vironment. The later gained and more trivial structures may 

 overlie and obscure the more ancient and fundamental ones, like 

 the later writings on an old palimpsest, but all are to be made 

 out by the skillful anatomist. As has been said, the line of 

 modification is not always a direct one, but it is often so de- 

 flected that structures which were primarily adapted to one fact 

 of environment may be modified secondarily to fit them for 

 others which are opposed and antagonistic to the first. The 



