342 THE VARIATION OF ANIMALS IN NATURE 



are expressions of periodic rhythms in the organism itself {cf. 

 p. 272). 



We have made the criticism (p. 328) against Haldane's 

 explanation of orthogenesis by means of a selective principle 

 that it is an ad hoc construction. The appeal to an internal 

 ' momentum ' seems, as we have admitted, open to the same 

 criticism, in so far as it postulates the existence of an activity 

 manifesting itself in long-sustained evolutionary series, the 

 only proof of the existence of which is the analogy with cer- 

 tain individual pathological phenomena and growth processes. 

 Viewed in this light neither of these explanations has much 

 to commend itself. The one fact that inclines us to favour 

 the second explanation is the impression we have gained that 

 however much the living organism is limited and confined by 

 its environment and the necessity of conforming thereto, it 

 still retains a measure of freedom. Monstrous structures often 

 seem void of adaptive significance ; but similar excesses in 

 behaviour are even more surprising. A single case may be 

 misleading, but it appears to be characteristic of much 

 of animal behaviour. We have in mind the facts relating 

 to the habits of the Australian Bower birds, which have 

 been studied by Barrett and Crandall (1932). The character 

 of the ' bowers ' made by these birds and the uses to which 

 they are put seem to be far in excess of the normal require- 

 ments of display and courtship and have little relation to 

 the survival requirements of the species. A somewhat 

 similar vagary of instinct is seen in some of the American 

 woodhewers (Homorus gutturalis). According to Hudson 

 (1924, p. 9), this bird, although only the size of a Missel 

 Thrush, makes a nest four or five feet, high with only a tiny 

 cavity inside. We suggest that, if such a capacity for gratui- 

 tous elaboration over and above the basic exigencies of mating 

 are manifested at the instinctive plane, the same freedom 

 may be found at the level of structure, and that many of the 

 phenomena of excessive growth and complexity are of the 

 same order. The value of such an analogy is admittedly con- 

 jectural. W r e think that it is not objectionable to argue that, 

 if some instincts have a latitude that transcends the exigen- 

 cies of mere survival value, as it is currently conceived, it is 

 not unlikely that the same is true of structural modifications. 

 It has to be freely granted that, even if the force of the 



