22 PROBLEMS OF GENETICS 



gesting that both may be one species. 18 Genetic tests, though as 

 yet imperfectly applied, make it almost certain that these inter- 

 grading forms are not in any true sense variations from either 

 species in the direction of the other, but combinations of elements 

 derived from both. 



The points in which very closely allied species are distin- 

 guished from each other may be found in the most diverse 

 features of their organisation. Sometimes specific difference 

 is to be seen in a character which we can believe to be important 

 in the struggle, but at least as often it is some little detail that 

 we cannot but regard as trivial which suffices to differentiate 

 the two species. Even when the diagnostic point is of such a 

 nature that we can imagine it to make a serious difference in the 

 economy we are absolutely at a loss to suggest why this feature 

 should be a necessity to species A and unnecessary to species B 

 its nearest ally. The house sparrow (Passer domesticus) is in 

 general structure very like the tree sparrow (P. montanus). 

 They differ in small points of colour. For instance montanus 

 has a black patch on the cheek which is absent in domesticus. 

 The presence in the one species and the absence in the other 

 are equally definite, and in both cases we are equally unable to 

 suggest any consideration of utility in relation to these features. 

 The two species are distinguished also by a characteristic that 

 may well be supposed to be of great significance. In domesticus 

 the two sexes are strongly differentiated, the cock being more 

 ornate than the hen. On the other hand the two sexes in mon- 

 tanus are alike, and, if we take a standard from domesticus, we 

 may fairly say that in montanus the hen has the colouration of 

 the male. It is not unreasonable to suppose that such a dis- 

 tinction may betoken some great difference in physiological 

 economy, but the economical significance of this perhaps im- 

 portant distinction is just as unaccountable as that of the seem- 

 ingly trivial but equally diagnostic colour-point. 



I have spoken of the fixed characteristics of the two species. 



18 As is well known, in an even more notorious example, he proposed to unite 

 Primula vulgaris, P. elatior, and P. acaulis, similarly relying on the existence of 

 "intermediates," which we now well know to be mongrels between the species. 



