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 he 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. R. 
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 ago 30 
I cited a certain class of cases in illustration of the fact that 
natural selection acts no n 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 wdiick 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 wdio 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., 1878, pp. 158—162. 
