the Mendelian principles were unknown to
Darwin. The time called for a bold pronouncement, and he made it, to
our lasting profit and delight. With fuller knowledge we pass once
more into a period of cautious expectation and reserve.

In every arduous enterprise it is pleasanter to look back at
difficulties overcome than forward to those which still seem
insurmountable, but in the next stage there is nothing to be stained
by disguising the fact that the attributes of living things are not
what we used to suppose. If they are more complex in the sense that
the properties they display are throughout so regular[71] that the
Selection of minute random variations is an unacceptable account of
the origin of their diversity, yet by virtue of that very regularity
the problem is limited in scope and thus simplified.

To begin with, we must relegate Selection to its proper place.
Selection permits the viable to continue and decides that the
non-viable shall perish; just as the temperature of our atmosphere
decides that no liquid carbon shall be found on the face of the earth:
but we do not suppose that the form of the diamond has been gradually
achieved by a process of Selection. So again, as the course of descent
branches in the successive generations, Selection determines along
which branch Evolution shall proceed, but it does not decide what
novelties that branch shall bring forth. "_La Nature contient le fonds
de toutes ces varietes, mais le hazard ou l'art les mettent en
oeuvre_," as Maupertuis most truly said.

Not till knowledge of the genetic properties of organisms has attained
to far greater completeness can evolutionary speculations have more
than a suggestive value. By genetic experiment, cytology and
physiological chemistry aiding, we may hope to acquire such knowledge.
In 1872 Nathusius wrote:[72] "Das Gesetz der Vererbung ist noch nicht
erkannt; der Apfel ist noch nicht vom Baum der Erkenntniss gefallen,
welcher, der Sage nach, Newton auf den rechten Weg zur Ergruendung der
Gravitationsgesetze fuehrte." We cannot pretend that the words are not
still true, but in Mendelian analysis the seeds of that apple-tree at
last are sown.

If we were asked what discovery would do most to forward our inquiry,
what one bit of knowledge would more than any other illuminate the
problem, I think we may give the answer without hesitation. The
greatest advance that we can foresee will be made when it is found
possible to connect