XV 
THE RELATION OF FACTS TO THEORIES 
309 
maintained by Natural Selection, and therefore it dis¬ 
appears,—the last therefore being one of the great 
points of dispute. Mr. Cunningham, on the other 
hand, considers that pigment is, or at least was 
primarily produced by the action of light on the 
skin, and that cave-inhabiting animals are pale- 
coloured because there is no light to stimulate the 
development of pigment. According to him, light 
and pigment are directly related; according to others, 
light is not the cause of pigmentation, it only puts in 
motion the machinery produced in the organism by 
Natural Selection. We have already seen by what 
beautiful experiments Mr. Cunningham has endea¬ 
voured to support and prove his position as to the 
relation between light and colour. 
4. Dr. Simroth's Theory. — Mr. Cunningham’s 
position may be taken as typical of that taken up 
by those who refer variation to the inherited and 
cumulative effect of environmental influences, but as 
an elaboration of the same principle we may take up 
a paper recently published by Dr. Heinrich Simroth. 
Dr. Simroth is well known, not only by his concrete 
researches, but as an ingenious and fertile theorist, 
and his present paper, though vague and mystical, 
has yet considerable interest, and to some extent 
may serve as a type of many of the most recent 
theories as to colour production. Dr. Sirnroth’s 
theory is, however, remarkable in displaying an 
absolute indifference to the facts of chemistry, which 
even among biologists is relatively rare. As papers 
of this kind are exceedingly difficult to interpret, 
it may be well to state clearly that although the 
following is an attempt to give a purely objective 
