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 
