nds may be, we can
only answer in a general way that, according to the general theory
of evolution, they must be such as are imposed by heredity, coupled
with the degree to which external conditions of life (and possibly
also use-inheritance) are capable, in given cases, of modifying
congenital characters. These are the only causes which the theory of
descent can consistently recognise as producing variations in
determinate directions.

(3) Inasmuch as variation presupposes the existence of parts that
vary, and inasmuch as the variation of parts can only be in the
alternative directions of increase or decrease around an average, it
follows that, in the first instance at all events, every variation,
if determinate, must be so only in one or other of these two
opposite directions.

(4) In as far as variations are summated in successive generations,
so as eventually to give rise to new structures, organs, mechanisms,
&c., natural selection is theoretically competent to explain the
facts, without our having to postulate the operation of unknown
causes producing variations in determinate lines,--or not further
than is stated in paragraphs 1 and 2.

(5) Nevertheless, it does not follow that there are not such other
unknown causes; and, if there are, of course the importance of
natural selection as a cause of adaptive modification would be
limited in proportion to their number and the extent of their
operation. But it is for those who, like the late Professors Asa
Gray and Naegeli, maintain the existence of such causes, to
substantiate their belief by indicating them.

Take any analogous case. The selective agency of specific gravity which
is utilised in gold-washing does not create the original differences
between gold-dust and dust of all other kinds. But these differences
being presented by as many different bodies in nature, the gold-washer
takes advantage of the selective agency in question, and, by using it as
a cause of segregation, is enabled to separate the gold from all the
earths with which it may happen to be mixed. So far as the objects of
the gold-washer are concerned, it is immaterial with what other earths
the gold-dust may happen to be mixed. For although gold-dust may occur
in intimate association with earths of various kinds in various
proportions, and although in each case the particular admixture whi