54 Darwin s Contribution. 



^ the particular biological application of this gen- 

 ^ eral doctrine, seeing that Lamarck and other 

 naturalists had maintained before Darwin that 

 our species of plants and animals were growths, 

 and not independent and immutable creations. 

 -N But Darwin s original contribution to the evolu- 

 ?T tionary theory was a demonstration of the mech 

 anism by which the development of species had 

 been effected. To take a specific example, he 

 undertook to show how it happened, by what 

 means it was brought about, that from one an 

 cestral species there could have descended, in the 

 course of thousands on thousands of generations, 

 four species so distinct as the horse, the ass, the 

 quagga, and the zebra, j^ 



In explaining how species originated, Darwin 

 got most help from the study of domesticated 

 animals and cultivated plants. The initial and 

 even fundamental fact of his whole theory is the 

 tendency of all living beings to vary ; and the 

 variations, which are generally minute and in 

 definite, are especially noticeable in our cultivated 

 plants and domesticated animals. Thus every 

 boy knows how much rabbits in a hutch differ 

 from one another in the hue of their fur, the 

 length of their ears, etc.; and anybody who has 

 paid the least attention to dogs, horses, cows, or 



