us has shown that, with the improved
races of the pig, the shortened legs and snout, the form of the
articular condyles of the occiput, and the position of the jaws
with the upper canine teeth projecting in a most anomalous manner
in front of the lower canines, may be attributed to these parts not
having been fully exercised.... These modifications of structure,
which are all strictly inherited, characterise several improved
breeds, so that they cannot have been derived from any single
domestic or wild stock. With respect to cattle, Professor Tanner
has remarked that the lungs and liver in the improved breeds 'are
found to be considerably reduced in size when compared with those
possessed by animals having perfect liberty;' ... The cause of the
reduced lungs in highly-bred animals which take little exercise is
obvious" (pp. 299-300). [And on pp. 301, 302 and 303, he gives
facts showing the effects of use and disuse in changing, among
domestic animals, the characters of the ears, the lengths of the
intestines, and, in various ways, the natures of the instincts.]

But Mr. Darwin's admission, or rather his assertion, that the
inheritance of functionally-produced modifications has been a factor in
organic evolution, is made clear not by these passages alone and by
kindred ones. It is made clearer still by a passage in the preface to
the second edition of his _Descent of Man_. He there protests against
that current version of his views in which this factor makes no
appearance. The passage is as follows.

"I may take this opportunity of remarking that my critics
frequently assume that I attribute all changes of corporeal
structure and mental power exclusively to the natural selection of
such variations as are often called spontaneous; whereas, even in
the first edition of the 'Origin of Species,' I distinctly stated
that great weight must be attributed to the inherited effects of
use and disuse, with respect both to the body and mind."

Nor is this all. There is evidence that Mr. Darwin's belief in the
efficiency of this factor, became stronger as he grew older and
accumulated more evidence. The first of the extracts above given, taken
from the sixth edition of the _Origin of Species_, runs thus:--

"I think there can be no doubt that use in our domestic animals has
strengthened and enlarged certain