138 DARWINISM 



Instability of Non-adaptive Characters. 



One very weighty objection to the theory that specific 

 characters can ever be wholly useless (or wholly uncon- 

 nected with useful organs by correlation of growth) appears 

 to have been overlooked by those who have maintained 

 the frequency of such characters, and that is, their almost 

 necessary instability. Darwin has remarked on the extreme 

 variability of secondary sexual characters — such as the horns, 

 crests, plumes, etc., which are found in males only, — the 

 reason being, that, although of some use, they are not 

 of such direct and vital importance as those adaptive 

 characters on which the wellbeing and very existence of the 

 animals depend. But in the case of wholly useless structures, 



skin lias grown so tough and hard that it hinders the increase in volume 

 which is inseparable from the growth of the animal. The casting of the 

 skin is induced by the formation on the surface of the inner epidermis, of a 

 layer of very fine and equally distributed hairs, which evidently serve the 

 purpose of mechanically raising the old skin by their rigidity and position. 

 These hairs then may be designated as casting hairs. That they are destined 

 and calculated for this end is evident to me from the fact established by Dr. 

 Braun, that the casting of the shells of the river cray-fish is induced in exactly 

 the same manner by the formation of a coating of hairs which mechanically 

 loosens the old skin or shell from the new. Now the researches of Braun and 

 Cartier have shown that these casting hairs — which serve the same purpose in 

 two groups of animals so far apart in the systematic scale — after the casting, 

 are partly transformed into the concentric stripes, sharp spikes, ridges, or 

 warts which ornament the outer edges of the skin-scales of reptiles or the 

 carapace of crabs." 1 Professor Semper adds that this example, with many 

 others that might be quoted, shows that we need not abandon the hope of 

 explaining morphological characters on Darwinian principles, although their 

 nature is often difficult to understand. 



During a recent discussion of this question in the pages of Nature, Mr. 

 St. George Mivart adduces several examples of what he deems useless specific 

 characters. Among them are the aborted index finger of the lemurine Potto, 

 and the thumbless hands of Colobus and Ateles, the " life-saving action " of 

 either of which he thinks incredible. These cases suggest two remarks. In 

 the first place, they involve generic, not specific, characters ; and the three 

 genera adduced are somewhat isolated, implying considerable antiquity and 

 the extinction of many allied forms. This is important, because it affords 

 ample time for great changes of conditions since the structures in question 

 originated ; and without a knowledge of these changes we can never safely 

 assert that any detail of structure could not have been useful. In the second 

 place, all three are cases of aborted or rudimentary organs ; and these are 

 admitted to be explained by non-use, leading to diminution of size, a further 

 reduction being brought about by the action of the principle of economy 



1 The Natural Conditions of Existence as they affect Animal Life, p. 19. 



