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 _specific_, characters; and the three genera adduced are somewhat isolated, implying considerable antiquity and the extinction of many allied forms.