The Nineteenth Century 235 



contained in a letter dated August 7, 1868, to G. H. Lewes (1817- 

 1878), the author of a history of philosophy,^^ No doubt Darwin 

 was at this time preparing the material for his third book on evolu- 

 tion, The Descent of Man and Selection in Relation to Sex (1871) • 

 In the letter Darwin wrote: 



I should just like to add, that we may understand each other, how I 

 suppose the luminous organs of insects, for instance, to have been de- 

 veloped; but I depend on conjectures, for so few luminous insects exist 

 that we have no means of judging, by the preservation to the present day 

 of slightly modified forms, of the probable gradations through which the 

 organs have passed. Moreover, we do not know of what use these organs 

 are. We see that the tissues of many animals, [as] certain centipedes in 

 England, are liable, under unknown conditions of food, temperature, 

 etc., to become occasionally luminous; just like the [illegible]: such 

 luminosity having been advantageous to certain insects, the tissues, I 

 suppose, became specialised for this purpose in an intensified degree; 

 in certain insects in one part, in other insects in other parts of the body. 

 Hence I believe that if all extinct insect-forms could be collected, we 

 should have gradations from the Elateridae, with their highly and con- 

 stantly luminous thoraxes, and from the Lampyridae, with their highly 

 luminous abdomens, to some ancient insects occasionally luminous like 

 the centipede. 



I do not know, but suppose that the microscopical structure of the 

 luminous organs in the most different insects is nearly the same; and I 

 should attribute to inheritance from a common progenitor, the simi- 

 larity of the tissues, which under similar conditions, allowed them to 

 vary in the same manner, and thus, through Natural Selection for the 

 same general purpose, to arrive at the same result. Mutatis jniitandis, I 

 should apply the same doctrine to the electric organs of fishes; but here 

 I have to make, in my own mind, the violent assumption that some 

 ancient fish was slightly electrical without having any special organs for 

 the purpose. 



In the Descent of Man and Selection in Relation to Sex (1874) , 

 Darwin wrote on the differences between the male and female glow- 

 worm as follows: 



The use of the bright light of the female glow-worm has been subject 

 to much discussion. The male is feebly luminous, as are the larvae and 

 even the eggs. It has been supposed by some that the light serves to 

 frighten away enemies, and by others to guide the male to the female. 

 At last, Mr. Belt ^s appears to have solved the difficulty: he finds that 

 all the Lampyridae which he has tried are highly distasteful to insec- 



^^ More letters of Charles Darwin, edited by Francis Darwin 1:306-307, letter to 

 G. H. Lewes, London, John Murray, 1903. 



**T. Belt, who wrote. The naturalist in Nicaragua, 1874. See p. 320. 



