Characters as Adaptive and Specific. 237 



full fertility between the form presenting them and 

 other forms which do not, then the latter fact will 

 usually prevent naturalists from constituting the well 

 differentiated form a species on grounds of its morpho- 

 logical features alone as, for instance, in the case of 

 our domesticated varieties. In short, the physiological 

 criterion has not been employed with sufficient close- 

 ness to admit of its being now comprised within any 

 practical definition of the term " species " if by this 

 term we are to understand, not what any one may 

 think species ought to be, but what species actually 

 are, as they have been constituted for us by their 

 makers. 



From all this it follows that the definition of the 

 term " species " on which Mr. Wallace relies for his 

 deduction with respect to specific characters, is the 

 definition No. 4. In other words, omitting his petitio 

 principii and his allusion to the test of fertility, the 

 great criterion in his view is the criterion of Heredity. 

 And in this all other evolutionists, of whatever school, 

 will doubtless agree with him. They will recognize 

 that it is really the distinguishing test between 

 " climatic varieties " and " true species," so that how- 

 ever widely or however constantly the former may 

 diverge from one another in regard to their peculiar 

 characters, they are not to be classed among the 

 latter unless their peculiar characters are likewise 

 hereditary characters. 



Now, if we are all agreed so far, the only question 

 that remains is whether or not this criterion of 

 Heredity is capable of supplying a basis for the 

 generalization, that all characters which have been 

 ranked as of .specific value must necessarily be 



