20 EVOLUTION AND DISEASE. 
Increase in the size of a part may arise from diminished 
use combined with irritation. For instance, nails, hoofs, 
and claws grow throughout life, and the wear and tear 
consequent on continued use is thus compensated. 
Should such parts be used less than usual, growth 
continues at the normal rate and the nails or hoofs 
become abnormally large and inconvenient. This form 
of enlargement is termed overgrowth, and is more liable 
to occur when diminished 
use is accompanied by 
irritation, as illustrated by 
the following specimen. 
It was a goat, confined for 
many weeks in a muddy 
paddock; on examining 
its feet I found them fur- 
nished with hoofs of great 
length, one measured 
thirty-six centimetres fol- 
FIG. Io, — An overgrown hoof in a lowing the curve, and its 
goat, which had lived many weeks = 
in a muddy paddock. It measures fellow twenty - five centi- 
thirty-six centimetres following the metres (fig. IO) The hoofs 
curve, ee 
of cows, horses, sheep, and 
deer become similarly overgrown when enclosed on 
marshy ground, dirty paddocks, or damp sheds. Similar 
conditions are not rare in our own species, for old, 
bedridden persons often have long toe-nails, some of 
them two or three inches long, thick and twisted like a 
ram’s horn. 
The relation between increase in the growth of nail 
or hoof in consequence of additional blood supply, is 
