RESPECTABLE AND DISREPUTABLE 

 RELATIVES 



THE case of the house-sparrow and the tree-sparrow 

 has been employed as one wherewith to challenge 

 the adequacy of the theory of natural selection. 

 There is a slight difference in the colouration of 

 the males, and whereas in the case of the house- 

 sparrow the male and female have distinct plumage, 

 in the case of the tree-sparrow the sexes are nearly 

 indistinguishable. " Such differences," says a 

 distinguished naturalist, " cannot be explained by 

 any theory of their survival value." The case is 

 not one of the strongest, for sex dimorphism, what- 

 ever may be its explanation, is undoubtedly a con- 

 siderable fact. But probably most Darwinians 

 would admit that there has been a great deal 

 too much of that sort of fine-spun argument which 

 detects survival value in fine shades of difference 

 in plumage. It is widely recognized that an 

 obvious mark of difference between species, as 

 in plumage, may have no direct significance what- 

 ever. Modifications rarely take place singly, and 

 there are certain modifications of parts which 

 always take place in groups or pairs. One of 

 them, and that not the patent one, may be the 

 change that counts. The stock case is that of 

 4 < 9 



