le in plants is influenced by the
conditions under which they grow.

Three accounts have been published in Eastern Prussia, of white and
white-spotted horses being greatly injured by eating mildewed and
honeydewed vetches; every spot of skin bearing white hairs becoming
inflamed and gangrenous. The Rev. J. Rodwell informs me that his father
turned out about fifteen cart-horses into a field of tares which in parts
swarmed with black aphides, and which no doubt were honeydewed, and
probably mildewed; the horses, with two exceptions, were chesnuts and bays
with white marks on their faces and pasterns, and the white parts alone
swelled and became angry scabs. The two bay horses with no white marks
entirely escaped all injury. In Guernsey, when horses eat fools' parsley
(_Aethusa cynapium_) they are sometimes violently purged; and this plant
"has a peculiar effect on the nose and lips, causing deep cracks and
ulcers, particularly on horses with white muzzles."[841] With cattle,
independently of the action of any poison, cases have been published by
Youatt and Erdt of cutaneous diseases with much constitutional disturbance
(in one instance after exposure to a hot sun) affecting every single point
which bore a white hair, but completely passing over other parts of the
body. Similar cases have been observed with horses.[842]

We thus see that not only do those parts of the skin which bear white hair
differ in a remarkable manner from those bearing {338} hair of any other
colour, but that in addition some great, constitutional difference must
stand in correlation with the colour of the hair; for in the
above-mentioned cases, vegetable poisons caused fever, swelling of the
head, as well as other symptoms, and even death, to all the white or
white-spotted animals.

* * * * *

{339}

CHAPTER XXVI.

LAWS OF VARIATION, _continued_--SUMMARY.

ON THE AFFINITY AND COHESION OF HOMOLOGOUS PARTS--ON THE VARIABILITY OF
MULTIPLE AND HOMOLOGOUS PARTS--COMPENSATION OF GROWTH--MECHANICAL
PRESSURE--RELATIVE POSITION OF FLOWERS WITH RESPECT TO THE AXIS OF THE
PLANT, AND OF SEEDS IN THE CAPSULE, AS INDUCING VARIATION--ANALOGOUS OR
PARALLEL VARIETIES--SUMMARY OF THE THREE LAST CHAPTERS.

_On the Affinity of Homologous Parts._--This law was first generalised by
Geoffroy Saint Hilaire, under the expression of _La loi de l'affinite de
soi pour soi_. It has been fully discussed and illustrat