re large, and three of
them, those which answer to the thumb and two following fingers in my
hand--are terminated by claws, while the fourth is enormously prolonged
and converted into a great jointed style. You see at once, from what I
have stated about a bird's wing, that there could be nothing less like
a bird's wing than this is. It was concluded by general reasoning that
this finger had the office of supporting a web which extended between it
and the body. An existing specimen proves that such was really the case,
and that the pterodactyles were devoid of feathers, but that the fingers
supported a vast web like that of a bat's wing; in fact, there can be no
doubt that this ancient reptile flew after the fashion of a bat.

Thus, though the pterodactyle is a reptile which has become modified
in such a manner as to enable it to fly, and therefore, as might be
expected, presents some points of resemblance to other animals which
fly; it has, so to speak, gone off the line which leads directly from
reptiles to birds, and has become disqualified for the changes which
lead to the characteristic organisation of the latter class.
Therefore, viewed in relation to the classes of reptiles and birds, the
pterodactyles appear to me to be, in a limited sense, intercalary forms;
but they are not even approximately linear, in the sense of exemplifying
those modifications of structure through which the passage from the
reptile to the bird took place.

III. THE DEMONSTRATIVE EVIDENCE OF EVOLUTION

The occurrence of historical facts is said to be demonstrated, when
the evidence that they happened is of such a character as to render the
assumption that they did not happen in the highest degree improbable;
and the question I now have to deal with is, whether evidence in favour
of the evolution of animals of this degree of cogency is, or is not,
obtainable from the record of the succession of living forms which is
presented to us by fossil remains.

Those who have attended to the progress of palaeontology are aware that
evidence of the character which I have defined has been produced in
considerable and continually-increasing quantity during the last few
years. Indeed, the amount and the satisfactory nature of that evidence
are somewhat surprising, when we consider the conditions under which
alone we can hope to obtain it.

It is obviously useless to seek for such evidence except in localities
in which the physical conditions