39(5 PHYSICAL CONDITIONS IN GENESIS OF SPECIES. 



- While so much is claimed by the writer as due to the direct action 

 of climatic causes, it is admitted also that habits and food and other 

 conditions of life than those resulting from climate have a marked 

 effect in determining modifications of form and color among animals. 

 A scarcity of a favorite kind of food will undoubtedly force species 

 to subsist upon the next best that offers, which may be so different as 

 to modify certain characters and fit the species to live upon the less 

 desired food. A change of food may lead to modification of denti- 

 tion, the muscles of mastication, and the organs of digestion and, 

 eorrelatively, of other organs or parts of the body; the modification, 

 however, arising simultaneously among all the descendants of the in- 

 dividuals thus driven to a change of diet, instead of appearing first in 

 a single individual and becoming perpetuated in its descendants alone. 

 Entomologists have found that, among insects of the same species, 

 the forced or voluntary use of different food plants gives rige to modi- 

 fications of color and structure, and hence result in what have been 

 termed phytophagic varieties or subspecies, and that man can also 

 effect such changes at will by simply changing the food of the species. 

 Again, the geological character of a country is well known to have a 

 marked effect upon the size ancl'color of animals inhabiting it, as is 

 strikingly illustrated among molluscous animals, whose abundance, 

 and even presence, is largely dependent upon the constituents of the 

 soil. Over regions of the United States, for example, where the un- 

 derlying rock is noncalcareous the species are both few in number 

 and sparsely re])resented, while in other regions, where limestone 

 abounds but which are in other respects essentially the same, the 

 species are far more numerous and far more abundantly represented. 

 In respect to the fresh-water mussels, those of the same species from 

 different streams are easily distinguishable by differences in the thick- 

 ness of the sliell, in color, shape, and ornamentation, so that the 

 character of the shells themselves affords a clew to the locality of their 

 origin. At some localities the species tend to become tuberculous 

 or spinous, this being particularly the case toward the southward ; at 

 other localities they acquire a very much thickened shell, or different 

 colors, the same characteristics appearing simultaneous!}' in quite 

 diverse species, and thus becoming distinctive of particular localities. 

 In regard to inammals, measurements of large series of the skulls of 

 minks, martens, squirrels, and other native species show that the 

 representatives of these species living in northern New England and 

 northeastern New York are smaller than the representatives of the 

 same species occurring in the limestone districts of Pennsylvania and 

 the States more to the westward, and tiie same is true of the different 

 kinds of domestic cattle. This is in opposition to the law of decrease 

 in size southward that elsewhere and generally chai:acterizes these 

 same species, and seems obviously related to the geological character 



