The microbe has a good deal to answer for.
On the case of the deterioration of horses in the Falkland Islands
after a few generations, Professor Weismann says:--
"In such a case we have only to assume that the climate which is
unfavourable, and nutriment which is insufficient for horses, affect
not only the animal as a whole but also its germ-cells. This would
result in the diminution in size of the germ-cells, the effects upon
the offspring being still further intensified by the insufficient
nourishment supplied during growth. But such results would not
depend upon the transmission by the germ-cells of certain
peculiarities due to the unfavourable climate, which only appear in
the full-grown horse."
But Professor Weismann does not like such cases, and admits that he
cannot explain the facts in connection with the climatic varieties
of certain butterflies, except "by supposing the passive acquisition
of characters produced by the direct influence of climate."
Nevertheless, in his next paragraph but one he calls such cases
"doubtful," and proposes that for the moment they should be left
aside.
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