be struck with surprise at what slight differences in the surrounding
circumstances govern the nature and severity of the complaints by which
man is at least temporarily affected.

The modifications as yet referred to have been extremely slight, and in
most cases have been caused, as far as we can judge, by equally slight
changes in the conditions. But can it be safely maintained that such
changed conditions, if acting during a long series of generations, would
not produce a marked effect? It is commonly believed that the people of the
United States differ in appearance from the parent Anglo-Saxon race; and
selection cannot have come into action within so short a period. A good
observer[676] states that a general absence of fat, {277} a thin and
elongated neck, stiff and lank hair, are the chief characteristics. The
change in the nature of the hair is supposed to be caused by the dryness of
the atmosphere. If immigration into the United States were now stopped, who
can say that the character of the whole people would not be greatly
modified in the course of two or three thousand years?

The direct and definite action of changed conditions, in
contradistinction to the accumulation of indefinite variations, seems
to me so important that I will give a large additional body of
miscellaneous facts. With plants, a considerable change of climate
sometimes produces a conspicuous result. I have given in detail in the
ninth chapter the most remarkable case known to me, namely, that in
Germany several varieties of maize brought from the hotter parts of
America were transformed in the course of only two or three
generations. Dr. Falconer informs me that he has seen the English
Ribston-pippin apple, a Himalayan oak, Prunus and Pyrus, all assume in
the hotter parts of India a fastigate or pyramidal habit; and this fact
is the more interesting, as a Chinese tropical species of Pyrus
naturally has this habit of growth. Although in these cases the changed
manner of growth seems to have been directly caused by the great heat,
we know that many fastigate trees have originated in their temperate
homes. In the Botanic Gardens of Ceylon the apple-tree[677] "sends out
numerous runners under ground, which continually rise into small stems,
and form a growth around the parent-tree." The varieties of the cabbage
which produce heads in Europe fail to do so in