480
by no means always for good. . . . Our business is to ask, not
what men have laid down, but how nature operates. Can a phae
nomenon have more than one cause, or can it not?”—(Scientific
Papers a?id Addresses, vol. ii., p. 721.)
“ Because one agency is proved to be a vera causa, it is not there
by proved that no other can by any possibility be competent simul
taneously to produce the same effect, whatever the schoolmen, with
the law of parsimony ringing in their ears, may have said to the
contrary.”—(Ibid. p. 83 g.)
He then proceeds to state certain cases in which a law
of parsimony has a place in nature, but he contends that
this law in no case excludes the possibility of a plurality of
causes :—
“ Many other instances of the law of parsimony might be given ;
but I know not of any which cannot be reduced under one or other
of these three heads ; I know of none, that is, which can be in any
way held to negative the tenability of a law of plurality of causes.”—
(A 7 2 3-)
If there is any sphere of nature in which a plurality of
causes is at work, it is surely in the colour and coloration of
animals. Heat and cold, summer and winter, light and dark
ness, plenty and famine ; the kind no less than the amount
of food ; physical, chemical, and organic conditions, the
inward organisation and the outward condition indicated by
that significant and much-embracing term “ climate”—all of
these affect the colours of animals ; and not unfrequently it
happens that the same colour is produced by two distinct
causes. Such being the case with respect to colour
generally, it should not surprise us to find that protective
colouring is also due to many causes as before explained.
Assuming that Natural Selection is one of the causes
of this phenomenon, it cannot be denied that it sometimes
occurs apart from Natural Selection—as, for example,
when the colour of the food-plant is seen through the
skin ; when the colour of the food-plant affects the blood,