d. They can show
that it is a matter of dispute whether some of these modified forms are
varieties or separate species. They can show, too, that the changes
daily taking place in ourselves--the facility that attends long
practice, and the loss of aptitude that begins when practice ceases--the
strengthening of passions habitually gratified, and the weakening of
those habitually curbed--the development of every faculty, bodily,
moral, or intellectual, according to the use made of it--are all
explicable on this same principle. And thus they can show that
throughout all organic nature there _is_ at work a modifying influence
of the kind they assign as the cause of these specific differences: an
influence which, though slow in its action, does, in time, if the
circumstances demand it, produce marked changes--an influence which, to
all appearance, would produce in the millions of years, and under the
great varieties of condition which geological records imply, any amount
of change.

Which, then, is the most rational hypothesis?--that of special creations
which has neither a fact to support it nor is even definitely
conceivable; or that of modification, which is not only definitely
conceivable, but is countenanced by the habitudes of every existing
organism?

That by any series of changes a protozoon should ever become a mammal,
seems to those who are not familiar with zoology, and who have not seen
how clear becomes the relationship between the simplest and the most
complex forms when intermediate forms are examined, a very grotesque
notion. Habitually looking at things rather in their statical aspect
than in their dynamical aspect, they never realize the fact that, by
small increments of modification, any amount of modification may in time
be generated. That surprise which they feel on finding one whom they
last saw as a boy, grown into a man, becomes incredulity when the degree
of change is greater. Nevertheless, abundant instances are at hand of
the mode in which we may pass to the most diverse forms by insensible
gradations. Arguing the matter some time since with a learned professor,
I illustrated my position thus:--You admit that there is no apparent
relationship between a circle and an hyperbola. The one is a finite
curve; the other is an infinite one. All parts of the one are alike; of
the other no parts are alike [save parts on its opposite sides]. The one
incloses a space; the other will not inclose a space though