CHAPTER XIII. 



ORIGIN OF SPECIES. 



WE have already seen reasons to conclude that the term 

 " species " must be regarded as being merely a convenient 

 abstraction by which we denote assemblages of individuals 

 having certain characters in common. We are thus led to 

 the belief that what naturalists ordinarily call " species " are 

 not unvarying and immutable quantities. We cannot, there- 

 fore, retain, in the sense in which he used it, the dictum of 

 Forbes, that " every true species presents in its individuals 

 certain features, specific characters, which distinguish it 

 from every other species ; as if the Creator had set an exclu- 

 sive mark or seal on each type." On the contrary, the 

 researches of Darwin, Wallace, and others, have compelled 

 the admission that all so-called species vary more or less, 

 and that these variations are sometimes so extensive that 

 the limits of specific distinctness are overstepped. Still, it 

 has not yet been demonstrated that these variations are 

 indefinite, either in direction or amount; and it remains, 

 therefore, possible that the process of specific variation is 

 bounded by fixed, if widely extended, limits, however pro- 

 bable the contrary may appear. 



It is impossible here to do more than merely indicate, in 

 the briefest manner, the two fundamental ideas which are at 

 the bottom of the leading theories which are entertained as 



