cance
they appear to be of no value in the life of the animal; indeed, they
are viewed as ornamental without regard to the fact that they are
microscopic and much too delicate to be visible to other animals of
their own species. It might, therefore, seem hopeless to show the
necessity for their existence on Darwinian principles, and to prove that
they are physiologically active organs. Nevertheless, recent
investigations on this point have furnished evidence that this is
possible.

"It is known that many reptiles, and above all the snakes, cast off the
whole skin at once, whereas human beings do so by degrees. If by any
accident they are prevented doing so, they infallibly die, because the
old 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 crayfish 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 _spe