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,