REPAIR IN EVOLUTION 89 



large variations. With regard to mesenteric bands the 

 same worker says : "In securing a proper fixation of the 

 abdominal viscera Nature calls to her aid processes which 

 are usually regarded as pathological." In this passage 

 " Nature " can obviously be translated into a series of 

 modifiable and transmissible phenomena. 



After reviewing phenomena such as these, the conclusion 

 seems inevitable that single small favourable variations 

 have not done the whole work of evolution. They may 

 play their part as correlated changes ; but they then take 

 their place in a series of which the causes can be recognized. 

 In combination with reasonable views of use and disuse, and 

 of increased or decreased blood-supply, they may, perhaps, 

 be held to explain such phenomena as the delicate 

 co-aptation of some cardiac valves. Their place in the 

 explanation of the phenomena of mimicry seems obvious. 

 But though they may help us to comprehend how tissues 

 become finished structures, if they are combined with the 

 results of functional energy, they yield no hint as to great 

 or decisive developments, and the mechanism involved in 

 them. If the reasons adduced for the thesis laid down carry 

 any weight, it is obvious that many, if not most, of the 

 really decisive variations in all internal structure depended, 

 and still depend, not on variations which can be called 

 favourable, but on those that for the major portion of the 

 organisms involved are directly disastrous ; not on varia- 

 tions which are small, but on those which are big 

 enough to be appreciable as the cause of immense 

 functional and structural results ; not on changes 

 which can in any sense be called spontaneous, by 

 which we may suppose are meant those no cause can 

 be assigned to, but on variations, which, though they 

 occurred ages ago, were obviously due to the very 



