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. Simroth'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 



