236 Darwin, and after Darwin. 



near allies, always produce offspring which are more or less sterile 

 inter se V 



From this definition the portion which I have 

 italicized must be omitted in the present discussion, 

 for the reasons already given while considering 

 definition No. 5. What remains is a combination of 

 Nos. 2 and 4. According to Mr. Wallace, therefore, 

 our criterion of a species is to be the heredity of 

 peculiar characters, combined, perhaps, with a more 

 or less exclusive fertility of the component individuals 

 inter se. This is the basis on which his generalization 

 of the utility of specific characters as necessary and 

 universal is reared. Here, then, we have something 

 definite to go upon, at all events as far as Mr. Wallace 

 is concerned. Let us see how far such a basis of 

 definition is competent to sustain his generalization. 



First of all it must be remarked that, as species 

 have actually been constituted by systematists, the 

 test of exclusive fertility does not apply. For my 

 own part I think this is to be regretted, because 

 I believe that such is the only natural and there- 

 fore the only firm basis on which specific dis- 

 tinctions can be reared. But, as previously observed, 

 this is not the view which has been taken by our 

 species-makers. At most they regard the physio- 

 logical criterion as but lending some additional weight 

 to their judgement upon morphological features, in 

 cases where it is doubtful whether the latter alone 

 are of sufficient distinctness to justify a recognition 

 of specific value. Or, conversely, if the morphological 

 features are clearly sufficient to justify such a recog- 

 nition, yet if it happens to be known that there is 

 1 Darwinism, p. 167. 



