MEMOIR. Ip 
segregate one or more distinct races out of its varieties in a third. These 
facts seem to establish principle No. 1, and I should think domestication and 
cultivation furnish many others. 
‘With regard to No. 2, Mr. Darwin quotes numerous instances, but 
generally with the qualifying suggestion that selection comes into play in 
almost every case. As natural selection is a/ways and of zecesszty in action, 
and the growth or atrophy of an organ a slow process requiring an immense 
number of generations to show conspicuous results, it must be impossible to 
disentangle the two principles in these cases. We should require to be shown 
(as you insist upon in other cases) a distinct case of growth or atrophy 
produced in a lifetime and transmitted to children. But any important 
degree of these requires many successive generations, and then, as in blind 
‘animals in caves, natural selection must come into play. 
‘‘ The same remarks apply to No. 3; but we have distinct facts to prove 
the propagation of acquired peculiarities. 1 think the fact of hereditary 
diseases proves a great deal, although you treat it slightly. It is true that a 
diseased species could not exist in nature. This is no matter; the fact 
remains that a peculiarity acquired in the lifetime of a parent is transmissible 
to its offspring. 
‘*T hope you will excuse my objecting to your conclusions regarding 
shaving and circumcision. These are slight mzutz/ations which seem, 
a priori, unlikely to be heritable: acquired modifications of the circulating 
fluids, which affect the skin, its clothing and colours, the glandular system, 
etc., etc., seem more likely to be transmissible to descendants. The action 
of climate, food, and habits on the individual during lifetime is, of course, 
undeniably great. So obvious is it that all naturalists have attributed 
local varieties to it. That a local variety has ever been formed solely by it I 
hold to be exceedingly improbable for the same reasons as above mentioned, 
namely, that it would take a great number of generations, and during the 
process natural selection wzwzs¢t come into play. 
‘Besides, the facts I have from nature of the segregation of three or 
more varieties in one locality, under uniform climatal conditions, prove how 
much more natural selection has to do with the matter than local physical 
conditions. The facts of hereditable diseases, however, go to show that the 
peculiarities acquired by the individual from action of climate, etc., may be 
transmitted to offspring; and this would explain many facts, such as the 
general similarity of foliage in certain localities and attitudes, prevalence of 
certain colours in insects in certain climates, and so forth. 
** All this goes to show that although natural selection is the main cause 
of the production of new races and species, yet external conditions have some 
effect in supplying it with variations to work upon, and giving a character to 
those variations ; and this is just Mr. Darwin’s position. 
‘* Nothing shows more strikingly how the two principles are intertwined in 
their workings than their effects on the fur of animals. It is admitted that 
heat and cold have a direct effect on the skin of sheep, and through it on 
their wool. On this account cold may have something to do with the thick fur 
of arctic animals. But the white colour of the fur of those which live on the 
snow cannot be at all the effect of snow, but must be wholly due to natural 
selection. ° 
