138 
DARWINISM 
CHAP. 
Instability of Non-adaptive Characters. 
One very weighty objection to the theory that specific 
characters can ever he wholly useless (or wholly uncon¬ 
nected with useful organs by correlation of growth) appears 
to have been overlooked by those who have maintained 
the frecpiency 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 has 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 
i The Natural Conditions of Existence as they affect Animal Life, p. 19. 
