356 EVOLUTION [CHAP. V 



Letter 267 seems to me well worth further development. I do not think 

 I have anywhere noticed the use of the eyebrows, but have 

 long known that they protected the eyes from sweat. During 

 the voyage of the Beagle one of the men ascended a lofty 

 hill during a very hot clay. He had small eyebrows, and his 

 eyes became fearfully inflamed from the sweat running into 

 them. The Portuguese inhabitants were familiar with this 



CJ 



evil. I think you allude to the transverse furrows on the 

 forehead as a protection against sweat ; but remember that 

 these incessantly appear on the foreheads of baboons. 



P.S. I have been greatly pleased by the notices in the 

 Nation. 



Letter 268 To A. Weismann. 



Down, May 1st, 1875. 



I did not receive your essay 1 for some days after your 

 very kind letter, and I read german so slowly that I have 

 only just finished it. Your work has interested me greatly, 

 and your conclusions seem well established. I have long 

 felt much curiosity about season-dimorphism, but never 

 could form any theory on the subject. Undoubtedly your 

 view is very important, as bearing on the general question 

 of variability. When I wrote the Origin I could not find 

 any facts which proved the direct action of climate and 

 other external conditions. I long ago thought that the 

 time would soon come when the causes of variation would 

 be fully discussed, and no one has done so much as you 

 in this important subject. The recent evidence of the 

 difference between birds of the same species in the N. and 

 S. United States well shows the power of climate. The 



has acted . . . has for several years seemed to me a somewhat less 

 important question than it seemed formerly, and still appears to most 

 thinkers on the subject. . . . The uses of the rattling of the rattlesnake 

 as a protection by warning its enemies and as a sexual call are not 

 rival uses ; neither are the high-reaching and the far-seeing uses of 

 the giraffe's neck ' rivals.' " 



1 Studien zur Desccndenz- Thcorie I . Ucber den Saison-Dimorphismus, 

 1875. The fact was previously known that two forms of the genus 

 Vanessa which had been considered to be distinct species are only 

 seasonal forms of the same species one appearing in spring, the 

 other in summer. This remarkable relationship forms the subject 

 of the essay 



