VARIATIONS IN SPECIES 111 



On his way back from Iceland, Newton paid a visit 

 to his friend H. B. Tristram (at that time rector of 

 Castle Eden), who had recently made two journeys to 

 Algeria and Tunis, where he had diligently collected 

 specimens of birds and reptiles. Among these he was 

 particularly interested by the desert-forms represented 

 in the large series of Larks or Chats. 



Generally the inhabiters of the desert took a duU 

 drab, but occasionally a warm or sand-coloured hue, 

 while those which did not dwell in the desert 'wore a 

 suit of much more decided and variegated tint. . . I 

 was at once reminded of what, in a less degree, I had 

 been shown and told the year before at Washington by 

 Professor Baird, who pointed out to me the variations 

 exhibited by examples of the same species of several 

 groups of North American birds, according as they came 

 from woodland, prairie, or elevated country. Among 

 all these were indications of a similar general law. The 

 woodland examples were the most highly coloured. 

 Those from the prairies were less deeply tinted ; while 

 those from the high plains — districts which, from what 

 I heard, seemed to approach in some degree the con- 

 dition of a desert such as is found in the Old World — 

 exhibited a fainter coloration. Here, then, was a sign 

 that like causes produced like effects even at the 

 enormous distances which separated the several localities. 

 The effects were plainly visible to the eye ; what were 

 the causes ? The only explanation offered to me by 

 Professor Baird, so far as I remember, was that the 

 chemical action of light, uninterrupted by any kind of 

 shade, produced the effect that was patent. With this 

 explanation, though it hardly seemed satisfactory, one 

 was fain to be content. 



It is thus apparent that Newton was ready and 

 anxious for a reasonable explanation of these problems, 

 and that he embraced the new teaching with enthusiasm 



