332 THE VARIATION OF ANIMALS IN NATURE 



(B) The following are examples of excessive complexity : 



Reptilia : 



Tooth-folds of Labyrinthodonts (Loomis, I.e.). 



Mollusca : 



Ammonite suture (auctt.). 

 Ennea, oral denticles (auctt.). 



Sponges : 



Excessive elaboration of spicules (Loomis, I.e.). 



Protozoa : 



Complexity of spines in Radiolaria (Loomis, I.e.). 



Excessive growth and elaboration of parts are manifested 

 in certain groups as a feature of sexual dimorphism. Various 

 appendages of male Crustacea, feathers and other parts of male 

 birds (or of the female in some cases), tusks and horns of 

 mammals are regularly enlarged for special purposes such as 

 coitus, fighting or display. It is true that in many such cases 

 the enlargement is far in excess of any imaginable exigencies 

 of courtship, competition, etc. {e.g. the remarkably heavy and 

 coiled horns of the male Ovispoli (Pamir or Marco Polo's Sheep) 

 (fig. 28) ). In others the appendages, etc., are enlarged in one 

 sex without any clearly ascertained function. The best studied 

 example of this is provided by the Fiddler Crab, Uca (Morgan, 

 Huxley), in which one of the chelae in the male is excessively 

 large and the other is normal. Pearse (1914) has studied the 

 behaviour of the Fiddler Crab and fails to find any definite 

 evidence as to its use beyond a vague suggestion that it is used 

 in display. It has also been suggested that it is used for 

 menacing other males or for stopping the entrance to the 

 burrows in which the animals live. When we find secondary 

 sexual characters of this kind ' running riot ' in size and com- 

 plexity it is always possible to refer them either to some ex- 

 ceptional but as yet unknown circumstance of courtship, etc., 

 or to the continuation by some equally unknown means of the 

 growth-processes originally stimulated by the sex hormones. 

 It is argued (cf. Fisher, 1930, pp. 136-137) that the original 

 impetus imparted by selection to some physiological activity 

 (such as the secretion and laying-down of keratin) may be 

 carried on after the particular adaptive end is attained. 



When we contrast the elaborate apparatus of display in the 



