2/2 EVOLUTION, OLD AND NEW. 



if the plant is able to stand the change and to perpe- 

 tuate itself for many generations, it will have become so 

 changed that botanists will class it as a new species." * 



"The same sort of process goes on in the animal 

 kingdom, but animals are modified more slowly than 

 plants." f 



The sterility of hybrids, to which Mr. Darwin devotes 

 a great part of the ninth chapter of his ' Origin of 

 Species,' \ is then touched on briefly, but sufficiently 

 as follows : 



"The idea that species were fixed and immutable 

 involved the belief that distinct species could not be 

 fertile inter se. But unfortunately observation has 

 proved, and daily proves, that this supposition is un- 

 founded. Hybrids are very common among plants, 

 and quite sufficiently so among animals to show that 

 the boundaries of these so-called immutable species are 

 not so well defined as has been supposed. Often, in- 

 deed, there is no offspring between the individuals of 

 what are called distinct species, especially when they 

 are widely different, and again, the offspring when 

 produced is generally sterile ; but when there is less 

 difference between the parents, both the difficulty of 

 breeding the hybrid, and its sterility when produced, are 

 found to disappear. In this very power of crossing we 

 see a source from which breeds, and ultimately species, 

 may arise." 



Mr. Darwin arrives at the same conclusion. He 

 writes : 



* ' Phil. Zool.,' torn. i. p. 80. t Pago 80. 



J Ed. 1876. ' PhiJ. Zool.,' torn. i. p. 81. 



