liv MEMOIR. 
and when I found such coming in contact with their supposed parents 
without intermarrying or reverting, I supposed the cause was the gradual 
and slow change in their constitutions which had brought them to a point 
when they were incapable of going back. 
‘But since I read Mr. Darwin’s book, I believe that natural selection 
has effected this change, operating both on structure and functions. But 
still the belief is strong that climatal and other causes have some slight 
effect. So slight, however, that of themselves they are unable to produce a 
race. Your letter shows me plainly that the effect must be yet slighter. I 
think the strongest case of the inability of local conditions acting directly 
to produce a race is that of the two races of man inhabiting the tropics of 
America and Africa. All facts go to show that the American race must 
have lived in Brazil many thousands of years, yet there is no approach to 
the negro. Yet the effect of exposure to the sun is to blacken the skin. For 
the nomad tribes of Amazon Indians are darker coloured than the agricul- 
tural, and the families of chiefs in Africa are lighter coloured than the rest of 
the tribe, etc., etc. 
‘‘My studies of those curious mimetic butterflies gave me convincing 
proof that local physical conditions do not directly produce a race, for there 
I see a species exhibiting nearly a dozen varieties in one limited spot, and 
four or five of them, as it were, in the act of being selected and segregated; 
the motive for such a process being also quite plain to detect. 
‘‘ A little more in corroboration of your views. On testing them by my 
case of the mimetic butterflies, I find they hold good. There is really no 
proof that the variable species originally sported in a different manner in one 
locality from what it did in another. Inappreciable variations or very small 
variations similar in all the localities, and natural selection drawing them 
out, as it were, would be sufficient to explain the whole case. 
‘‘ The elimination of intermediate varieties formerly, and of the occasional 
offspring of cross-marriages between the divergent selected forms in one limited 
district, must have been, and must be now, very stringent and perpetually 
acting ; probably, however, there is a moral barrier preventing frequent inter- 
marriages, although there is not a geographical one; in other words, the 
selected and now somewhat widely diverged varieties refuse to intermarry. 
‘“ Now for a few cases which seem to show that the modifications induced 
by direct action are propagated. 
‘© A few sheep have been imported into the Amazons region from Portugal. 
The wool nearly all falls off, and the young at about two or three months old 
(when alone I have noticed them) have a thin coat of straight wool, more 
like hair than wool. I know this is an imperfect observation. I did not 
notice that #azy, or a tendency to hair, was produced in the lifetime of a 
sheep, and I did not see lambs when first born, to prove that they have this 
modified coating at their birth. The sheep are only kept by twos and threes, 
as pets, and have not been in the country for many generations, so that I do 
not think selection, artificial or natural, has operated. 
‘‘ Dogs become lop-eared in consequence of the relaxation of muscle from 
disuse; puppies are born lop-eared. 
‘‘Mr. Darwin’s case of tame ducks having less developed wing-bones, 
from disuse, seems to come under this head, but unconscious or natural 
