SELECTION 459 



attractive males, added to their beauty or other attractive 

 qualities " (Descent of Man, 2nd Ed., 1888, Vol. L, p. 326). 

 In many animals, at diverse levels of organisation, there is 

 an elaborate courtship-ceremonial, allied, according to Groos, 

 to play. It is sometimes on both sides ; it is usually for the 

 most part on the male's side. It includes a manifold display 

 of decorations, colours, agility, and vocal powers. Darwin's 

 theory in this connection was simply this: if there are rival 

 males, and if they are unequally endowed with structural 

 and emotional equipment, or with the capacity of using this 

 to advantage, there will be preferential mating on the 

 female's part, and, other things equal, there will be a selec- 

 tion of the type of male most successful as a suitor. It 

 is the female who sifts, but the logic of the process is the 

 same as in natural selection. 



(e) It is conceivable that pronounced and persistent 

 differential mating might lead not merely to the establish- 

 ment and augmentation of characters determining the result 

 of the contest or the courtship, but also to a process of physio- 

 logical and psychological l isolation ' (narrowing of the range 

 of inter-crossing), and thus to an accentuation of the apart- 

 ness of a species as regards crossing with related neighbour- 

 species (see Karl Pearson, Grammar of Science, 2nd Ed., 

 1900, p. 418). 



(/) At this point attention may be directed to the impor- 

 tant contributions to the natural history of mating to be 

 found in H. Eliot Howard's monumental British Warblers 

 (1907-1915). We venture to think that this acute and sym- 

 pathetic observer exaggerates the instinctive at the expense 

 of the intelligent element in the behaviour of birds, and 

 that he is unnecessarily antagonistic to Darwin's theory 

 of sexual selection, but his work is a rich treasure-house 



