38 THE PHYSICAL KINSHIP 



offspring; Heredity, which tends to perpetuate 

 peculiarities by causing offspring to resemble 

 more or less the characters of their parents ; and 

 Environment, which determines the character of 

 the selections, are the three factors, and the only 

 three factors, in organic evolution. 



IX. The Evidences of Organic Evolution. 



That the forms of life to-day found on the earth 

 have come into existence by the evolution of the 

 more complex forms from the simpler, and of these 

 simpler forms from still simpler, through the ever- 

 operating law of Selection, is a necessary conclusion 

 from the following facts : 



I. The existence in the animal world of all 

 grades of structures, from the humblest possible 

 protozoan, whose body consists of a single simple 

 speck, to the most powerful and complex of 

 mammals. There are estimated to be something 

 like a million species of animals living on the 

 earth to-day. There may be several times this 

 number. These species are linked together by 

 millions of varieties, and are so related to each 

 other that they may be all gathered together into 

 various genera ; these genera may be grouped into 

 families, the families into orders, and the orders 

 into seven or eight great primary phyla. By 

 taking existing species and adding to them the 

 extinct species of the rocks, and placing them all 

 according to their structural affinities, it is pos- 

 sible to arrange them in the form of a tree with 

 the various phyla, orders, families, genera, and 



