ORGANIC EVOLUTION 



267 



horses bred for draft or for speed ; peas selected for color of 

 flower or for palat ability of seed, etc. And in his mind's 

 eye he saw nature producing like results by the removal of 

 the unfit and the preservation of those best suited to her 

 conditions. So, he called the process natural selection. 



The theory of natural selection is based on four facts: 

 i) Organisms vary; 2) In every species more young are 

 produced than can possibly survive; 3) Offspring tend to 



Fig. 162. Three sassafras leaves from the same tree. 



resemble parents ; and 4) There exists competition between 

 the members of the earth's population. 



Inheritance w411 be considered in the next chapter. Let 

 us here examine the other three classes of facts severally. 



Variation. — Animals and plants vary. No two persons 

 look alike, nor do the individuals of any species, on suffi- 

 ciently close acquaintance. The careful shepherd knows 

 his sheep as individuals, and it is only to the casual observer 



