vI ORIGIN OF THE COLOUR-SENSE 411 
that visual organs might exist which would differentiate what 
we term colour by delicate gradations of some one charac- 
teristic neutral tint. Now such a capacity of vision would 
be simple as compared with that which we actually possess, 
which, besides distinguishing infinite gradations of the quan- 
tity of light, distinguishes also, by a totally distinct set of 
sensations, gradations of qualify, as determined by differences 
of wave-lengths or rate of vibration. At what grade in 
animal development this new and more complex sense first 
began to appear we have no means of determining. The 
fact that the higher vertebrates, and even some insects, dis- 
tinguish what are to us diversities of colour by no means 
proves that their sensations of colour bear any resemblance 
whatever to ours. An insect’s capacity to distinguish red 
from blue or yellow may be (and probably is) due to percep- 
tions of a totally distinct nature, and quite unaccompanied by 
any of that sense of enjoyment or even of radical distinctness 
which pure colours excite in us. Mammalia and birds, whose 
structure and emotions are so similar to our own, do probably 
receive somewhat similar impressions of colour; but we have 
no evidence to show that they experience pleasurable emotions 
from colour itself when not associated with the satisfaction of 
their wants or the gratification of their passions. 
The primary necessity which led to the development of 
the sense of colour was probably the need of distinguishing 
objects much alike in form and size, but differing in important 
properties, such as ripe and unripe, or eatable and poisonous 
fruits, flowers with honey or without, the sexes of the same 
or of closely allied species. In most cases the strongest con- 
trast would be the most useful, especially as the colours of 
the objects to be distinguished would form but minute spots 
or points when compared with the broad masses of tint of 
sky, earth, or foliage against which they would be set. 
Throughout the long epochs in which the sense of sight 
was being gradually developed in the higher animals, their 
visual organs would be mainly subjected to two groups of 
rays—the green from vegetation, and the blue from the sky. 
The immense preponderance of these over all other groups of 
rays would naturally lead the eye to become specially adapted 
for their perception; and it is quite possible that at first 
