340 THE VARIATION OF ANIMALS IN NATURE 



be at the offset conditioned by some physiological adaptation ? 

 The argument would run thus : We often find the males of 

 a species possessing some excessively developed structure. 

 The dimorphism suggests either that the excessive growth is 

 based on some functional peculiarity which it has outstripped 

 or that it is a by-product of some abnormal glandular activity. 

 When the excessive growth is not associated with sexual 

 dimorphism, but occurs in both sexes, is it not likely to have 

 similarly originated in some adaptive phenomenon or to be 

 due to some by-product of physiological activity ? The 

 reasoning is merely analogical ; but it is at least suggestive. 

 Moreover, among the cases of sexually differentiated structures 

 there are many (combs of fowls, horns in ruminants) the 

 growth of which is definitely known to be influenced by 

 specific secretions. Furthermore, it is well known that irregu- 

 larities of growth are associated with abnormal conditions of 

 the thyroid and pituitary. There is little doubt, then, that 

 a physiological basis exists for such growth principles. Lastly, 

 individual growth disharmonies similar to the characters which 

 distinguish genera and species are well known, and Bland 

 Sutton (1890) has collected a large number of examples 

 illustrating this parallelism. The role of such physiological 

 and pathological factors as causing ' momentum ' in evolution 

 has been discussed and emphasised by Dendy (191 1), Keith 

 (1922), and Lang (192 1). 



We admit that the case so far is analogical. We have no 

 evidence that in a given instance an evolutionary history is 

 determined by such causes. But the analogy is so striking 

 that it calls for serious notice. 



Of course, even if some disturbance of the normal growth 

 processes is at work, we have still to account for the origin 

 of the disturbance, for the removal of the normal inhibitions. 

 For this we can but make suggestions by analogy with the 

 known effects of the absence of certain genes, particular en- 

 vironmental effects or pathological disturbances. The case 

 has been well argued by Lang {I.e. p. xiv). It may be con- 

 tended that the apparent physiological impetus is merely 

 the effect of selection on the appropriate physiological basis. 

 There is, however, no actual evidence in support of this 

 suggestion. 



(b) If the facts and arguments presented in (a) seem to 



