lungs; but no reason has been given for the evolution of bone in place of
cartilage or for the various kinds of scales. Professor Houssaye, on the
other hand, believes that the number and position of fins is adapted to
the shape and velocity of movement of each kind of fish.

If we turn to other groups of animals we find everywhere similar evidence
of the distinction between adaptive and non-adaptive characters. Birds are
adapted in their whole organization for flight, the structure of the wing,
of the sternum, breast muscles, legs, etc., are all co-ordinated for this
end. But how do we know that feathers in their origin were connected with
flight? It seems equally probable that feathers arose as a mutation in
place of scales in a reptile, and the feathers were then adapted for
flight. Nothing shows the distinction better than convergent adaptation.
Owls resemble birds of prey in bill and claw and mode of life, yet they
are related to insect-eating swifts and goat-suckers and not to eagles and
hawks. Swifts and swallows are similar in adaptive characters, but not in
those which show relationship. It may be said that the characters believed
to show true affinities were originally adaptive, but we do not know this.
Similarly, in reptiles the Chelonia are distinguished by the most
extraordinary union of skin-bones and internal skeleton enclosing the body
in rigid armour: it may be said that the function of this is protection,
that it is adaptation, and can be explained by natural selection, but the
adaptation in this case is so indefinite that it is difficult to be
convinced of it.

Systematists have always distinguished between adaptive characters and
those of taxonomic value--those which show the true affinities--and they
are perfectly right: also they have always distrusted and held aloof from
theories of evolution which profess to explain all characters by one
universal formula. In my opinion, those who, like Weismann, consider all