My dear Sir

Many thanks for your photograph.¹

The keeper in the Zoolog. Gardens is quite right with regard to the difference in color in the sexes of Rattlesnakes.² This case is perfectly analogous to that of the common viper, which I mentioned to you the other day. I do not know of any sexual structural (external) difference in snakes, except that the tail of males is generally longer & more slender.

Male snakes are always smaller, & generally more brightly coloured, & with the markings more distinctly defined than females.³

As regards other Reptiles, I will mention some remarkable cases in Indian species, & thinking that you are a subscriber to the Ray Society, & have at hand my book on Indian Reptiles, I may refer you to the pages.⁴ If you have not the book, let me know it, & I will supply you with further details.

Page 130 & 131. Pl. 13. figs F & G. Ceratophora stoddartii & aspera. The rostral appendage is perfectly analogous to the comb of a cock, being but little developed in the female & young.⁵

Page 132. Cophotis   The dorsal crest is much more developed in the male than in the female.⁶ This is one of numerous similar instances, all of which show that in Lizards with a dorsal crest, this is more developed in the male. Iguana is such an example.

p. 135. pl. 14. fig. A. Sitana is a lizard with a very large gular pouch, entirely absent in the female. There are other Lizards in which both sexes are equally provided with a gular pouch.⁷

p. 143 | Calotes nigrilabris offers a remarkable instance of sexual difference in coloration of the head & body.⁸

Sexual differences in coloration of snakes I have mentioned particularly in Tragops dispar p. 304, Dipsas cynodon p. 308, Trimeresurus erythrurus p. 386.⁹

As regards Frogs, the case of Megalophrys montana p. 413–414 is very striking; the male has a rostral appendage (skinny), absent in the female; it has a pointed appendage above the eye, much less developed in the female; & some tubercles on the back, also absent in the female.¹⁰

So much for today. If I come across other cases, I shall note them. I think, to appreciate fully the instances mentioned, you ought to look at the specimens when you come to the Museum.¹¹

Many thanks for the promised book, on Variation; you underrate me when you say, that it will contain very little which can interest me.¹²

Yours sincerely | A Günther