OTHER THEORIES OF EVOLUTION 333 



male Argus Pheasant or the Peacock and the unostentatious 

 structure and subdued colour of other equally successful 

 vertebrates, we cannot but conclude, if the display of the former 

 is a necessary part of the mating behaviour, either that it 

 must be evoked by very exceptional emotional conditions, or 

 that it has no adaptive significance as far as reproduction is 

 concerned (see p. 292). 1 



In considering the various explanations of these growth 

 phenomena, it will be as well to bear in mind the following 

 points : 



(1) In many groups of animals individual species, genera 

 or families tend to outrun the normal size of the group. The 



Fig. 28. — Horns of Ovis poll (male). 

 (British Museum (Natural History).) 



usual adaptive explanations of such excessive bulk as is seen 

 in the Greenland Whale, the Giant Squids, etc. (viz. that large 

 size is advantageous), are not satisfactory. One can hardly 

 imagine that sedentary organisms like the Giant Shipworm and 

 Giant Clam can derive any benefit from their excessive size. 

 As Lang (I.e.) points out, in Hippurites the protection offered 

 by the thickness of the under-valve is far in excess of any 

 reasonable demand for safety against predators. 



(2) The assessment of any structure as ' abnormal ' or 

 ' extravagant ' is determined by purely arbitrary standards. 

 At the best we can take very extreme cases as ' abnormal.' 



(3) Some structures seem to us at first sight to be so 



1 Hingston (1933) in an interesting book (the main argument of which is 

 open to criticism) supplies much evidence tending to show that the display of 

 various male birds is entirely disregarded by the female. 



