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above the centre of the cluster was equal to that at a little distance 
below ; he also found that the law of diminution of density on 
receding from the cluster was given by a certain mathematical 
expression, to which, however, I need here make no further reference. 
A similar law obtains with reference to the circumference of the 
chest ; and one may assume, with some confidence, that, under 
normal conditions, the variation of any organ in the same species 
may be symmetrically grouped about a centre of greatest density, as 
above explained.”—(Nature, vol. viii., p. 505.) 
Mr. Wallace enunciates the same principle in the follow 
ing passage :— 
“ If now we consider the population of a species with regard to 
any particular character or combination of characters, we may divide 
it into three groups—a central group in which the mean or average 
development prevails with little variation, one in which the character 
is excessively, and one in which it is little developed. These groups 
would not be of equal extent, the central portion—that in which the 
mean characteristics prevailed—being in accordance with the law of 
averages, much more numerous than the extremes ; perhaps twice or 
even three times as great as either of them, and forming such a series 
as the following :—Maximum development io, mean 30, minimum 10. 
These figures, whatever their exact proportions, would probably be 
pretty constant, for we have no reason to believe that the mean 
characters, or the amount of variation of a species, change materially 
from year to year or from century to century ; and we may therefore 
look upon the central and most numerous group as presenting the 
typical form of the species, being that which is best adapted to the 
conditions in which it has actually to exist, while the extremes, being 
less perfectly adapted, are continually weeded out by natural 
selection.”—(Nineteenth Century, vol. vii.,ftp. 100-1.) 
And yet Mr. Wallace elsewhere arrives at a different 
result. He says :— 
“ Mr. Allen also gives full details as to the variation of colour and 
marking, showing that these are not less striking than those of size 
and proportions ; but the most important thing for us in regard to 
the question we are discussing is the amount of simultaneous varia 
tion of the same kind that is constantly occurring. To determine 
this I formed diagrams, in which each individual was represented 
by a spot placed on a horizontal line at a point determined by its 
actual dimensions. It would have been antecedently expected that 
the great bulk of the spots would be crowded together about a point 
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