41 o POPULAR SCIENCE MONTHLY. 



recting dull proof-sheets. When I return to the work I shall find it 

 much better done by you than I could have succeeded in doing. 



It is curious how we hit on the same ideas. I have endeavoured to 

 show in my MS. discussion that nearly the same principles account 

 for young birds not being gaily coloured in many cases, but this is too 

 complex a point for a note. 



On reading over your letter again, and on further reflection, I do 

 not think (as far as I remember my words) that I expressed myself 

 nearly strongly enough on the value and beauty of your generalisa- 

 tion, viz., that all birds in which the female is conspicuously or 

 brightly coloured build in holes or under domes. I thought that this 

 was the explanation in many, perhaps most cases, but do not think I 

 should ever have extended my view to your generalisation. Forgive 

 me troubling you with this P.S. 



To A. It. Wallace. 



Down, May 5th [1867]. 

 The offer of your valuable notes is most generous, but it would 

 vex me to take so much from you, as it is certain that you could work 

 up the subject very much better than I could. Therefore I earnestly, 

 and without any reservation, hope that you will proceed with your 

 paper, so that I return your notes. You seem already to have well 

 investigated the subject. I confess on receiving your note that I felt 

 rather flat at my recent work being almost thrown away, but I did not 

 intend to show this feeling. As a proof how little advance I had made 

 on the subject, I may mention that though I had been collecting facts 

 on the colouring, and other sexual differences in mammals, your ex- 

 planation with respect to the females had not occurred to me. I am 

 surprised at my own stupidity, but I have long recognised how much 

 clearer and deeper your insight into matters is than mine. I do not 

 know how far you have attended to the laws of inheritance, so what 

 follows may be obvious to you. I have begun my discussion on sex- 

 ual selection by showing that new characters often appear in one sex 

 and are transmitted to that sex alone, and that from some unknown 

 cause such characters apparently appear oftener in the male than in 

 the female. Secondly, characters may be developed and be confined 

 to the male, and long afterwards be transferred to the female. 

 Thirdly, characters may arise in either sex and be transmitted to both 

 sexes, either in an equal or unequal degree. In this latter case I have 

 supposed that the survival of the fittest has come into play with female 

 birds and kept the female dull-coloured. With respect to the absence 

 of spurs in the female gallinaceous birds, I presume that they would be 

 in the way during incubation; at least I have got the case of a Ger- 

 man breed of fowls in which the hens were spurred, and were found 



