488 LIFK : OUTLINES OF GENERAL BIOLOGY 



level of differentiation of, say. the stag's antlers and the decorations 

 of the Bird of Paradise, could be gradually raised generation after 

 generation by such selection as combats and preferential mating 

 respectively afford, (b) In some cases of pronounced sex-dimorphism 

 there is no evidence either of preferential mating or of combats, 

 and stibsidiary hypotheses have to be invoked. Thus wc have 

 (iiinther's suggestion that masculine characters have their justifica- 

 tion as a means of "bluffing" enemies, (f) Statements in regard to 

 serious disproportion in the number of the two sexes must be taken 

 critically, having broken down in several cases. And it is plain that 

 the value of the selectionist interpretation depends largely on the 

 evidence that considerable numbers of the less attractive or less 

 well-equipped males are either left unmated, or have less numerous 



Fig. 69. 



The Male Calling Crab or Fiddler Crab (Gelasimus). From a specimen. The 

 male is marked by the exaggerated great claw or forceps (F), which 

 may be as big as the rest of the body. The corresponding appendage, Fi, 

 to the left, is small. The male brandishes his brightly coloured forceps 

 when courting the female. The eyes (E) are on long stalks. The four 

 pairs of walking-legs are as usual. 



and successful families as the result of their matings. (d) As Darwin 

 himself hinted, there is much reason to think that the female who 

 has to be wooed surrenders herself not to the male who has a par- 

 ticular character in special excellence, but to the one whose tout 

 ensemble has most successfully excited her .sexual interest. 



Cunningham's Thkory. — In his interesting Sexual Dimorphism 

 Mr. J. T. Cunningham argues in support of a Lamarckian interpre- 

 tation. "In either sex unisexual characters have, as a general rule, 

 .some function or importance in the special habits or conditions of 

 life of the sex in which they occur." "But the important truth, 

 which appears to have been generally overlooked, is that in the case 

 of each special organ its sjx^cial employment subjects it to .special, 

 usually mechanical, irritation or stimulation, to which other organs 

 of the body are not subjected. Every naturalist and every physio- 



