The Presidential Address.
87
'Variation under Nature,' in a work which was promised to
follow his Variation of Animals and Plants under Domesti-
cation '; but it is well known that his system never com-
pletely rallied from the effects of the voyage of the 'Beagle,'
and his work, herculean though it may be in the eyes of his
contemporaries, becomes greatly enhanced in wonder when
we consider that for years he laboured bravely under the
most distressing physical disadvantages. But although the
great task remains uncompleted in its details, much light was
thrown upon this important subject by Darwin in his other
works, and it has since been discussed in many able treatises
emanating from the German school of Darwinians. With
respect to the extent and origin of variability, I cannot do
better than refer you to a very valuable paper by Mr. A. B.
Wallace, in the 'Nineteenth Century' for January, 1880.
This question of the origin of variability, however important
on other grounds, is in fact of secondary consideration as far
as concerns the Darwinian theory proper. Ten years ago30
I cited a certain class of cases in illustration of the fact that
natural selection acts upon such variations as arise, with
entire disregard to the causes of these variations. When,
indeed, we look upon an individual organism, as we now
must, as a being in which is epitomised the hereditary
tendencies of a long chain of ancestors which have at various
periods been exposed to the most diverse conditions, I cannot
but agree with those who maintain that the problem should
be inverted, and that it is not variability but constancy
which demands an explanation. "Does any one ask for a
reason why no two gravel-stones or beach-pebbles, or even
grains of sand, are absolutely identical in size, shape, surface,
colour, and composition ? When we trace back the complete
series of causes and forces that have led to the production of
these objects, do we not see that their absolute identity
would be more remarkable than their diversity'? So, when
we consider how infinitely more complex have been' the
forces that have produced each individual animal or plant,
and when we know that no two animals can possibly have
30 Proc. Zool. Soc, 1873, pp. 153—162.