78 The Presidential Address.
based, as previously stated, upon certain indisputable facts,
viz., the high rate of multiplication of living beings, the
struggle for existence, change of environment, and the
principle of heredity. Every species is, on this view,
genetically descended from some previously existing species;
and since the changes of external conditions take place on
the whole with extreme slowness, the process of modification
must have proceeded with corresponding slowness. It is of
the utmost importance to bear in mind that, in admitting
the Darwinian theory, we for ever exclude the possibility of
any "innate tendency" to change in species ; the modifica-
tion of living forms is mainly brought about by external
factors, and we cannot regard a species as a production
endowed with a predisposition to grow into some other form
by virtue of an internal law of growth making itself manifest
only with the lapse of time. As long as the external con-
ditions remain unchanged the species remains fixed. Para-
phrasing Newton's first law of motion, we might say that a
species perseveres in its existing state unless acted upon by
external forces. Whether the modification of species has
always proceeded at the same rate is perhaps an open
question, but considering the vicissitudes both astronomical
and geological through which our earth has passed, and
supposing that life appeared as soon as permitted by the
temperature of the globe cooling down from a molten con-
dition, it seems to me that change, both geological and
organic, may have taken place at a greater rate in past ages
than at the present time, in which case the slow present rate
of transformation may be no absolute measure of the past
rate of modification, and the large demands upon time made
by many supporters of the descent theory may permit of
being more or less curtailed.
The classification of animals and plants into species,
genera, orders, classes, &c, according to their "affinities,"
now, as in pre-Darwinian times, continues to occupy the
attention of large numbers of our working naturalists, but in
the light of the theory of descent it is seen that this arrange-
ment into groups within groups is no less than an attempt to