Natural Selection of Species. 105 



CHAPTER XVI. 



NATURAL SELECTION OP SPECIES. 



Geological history shows and human observation during historical 

 times agrees that there is a constant modification of the species of 

 plants and animals. As one generation dies its heirs and successors are 

 certain to differ in some respect from it, and the accumulations of dif- 

 ferences repeated and added to each other finally amount to enough to 

 justify the distinction of specific names. 



It is not difficult to point out general causes that would tend to pro- 

 duce changes in organization, although it is not often possible of course 

 to designate in detail the action of these causes in particular cases. 



Animal life depends on plant life. Certain kinds of animals require 

 herbage of a bulky and abundant kind. Our American Bison swarmed 

 in great herds over the prairies of the west because among other things 

 it had a vast abundance of nutritious grass. Wolves likewise were 

 abundant there because the casualties among the Bisons furnished them 

 with plenty of animal food. The grass in its turn depended on the 

 quality of the soil and the amount of the rainfall. 



The rainfall depends on the topography of the country. A different 

 arrangement of mountain ranges and plains might have rendered the 

 region rainless and barren like Sahara or Gobi, or subject to occasional 

 protracted drouths fatal to herds depending on grass. The topography 

 of the country in turn was determined by dynamical causes that were in 

 action probably hundreds of millions of years ago, by which certain 

 parts of the earth's crust became wrinkled into ridges and mountain 

 chains and other parts elevated above the sea as plains. These dynami- 

 cal forces in turn are traceable at last to the simple affections of ele- 

 mentary matter, of which gravitation is the first, if not the representa- 

 tive of all. Gravitation produced the motion of matter toward the 

 common centers of systems, resulting in its accumulation in the form of 

 globes, and in the evolution of heat, light and magnetism, if not chem- 

 ical activity also. At any rate these causes, primary and secondar} T , 

 have made the world as it is with seas, lakes, rivers, mountains, plat- 

 eaus, low-lands and marshes, and established upon it every variety of 

 temperature and many degrees of moisture. 



Now without going into details it is plain that these general aspects 

 of nature must possess an influence over the organisms within their 

 reach. The diminution of heat on the approach of winter allows the 

 polar cap of snow to spread itself 25 degrees towards the equator, and 

 imposes upon the vegetable and animal kingdoms in these latitudes 



