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luminous organs of insects, for instance, to have been developed ; Letter 223 

 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, become 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 similarity 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 mutandis, 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. It has 

 been stated on evidence, not trustworthy, that certain reptiles 

 are electrical. It is, moreover, possible that the so-called 

 electric organs, whilst in a condition not highly developed, 

 may have subserved some distinct function : at least, I think, 

 Matteucci could detect no pure electricity in certain fishes 

 provided with the proper organs. In one of your letters 

 you alluded to nails, claws, hoofs, etc. From their perfect 

 coadaptation with the whole rest of the organisation, I cannot 

 admit that they would have been formed by the direct action 

 of the conditions of life. H. Spencer's view that they were 

 first developed from indurated skin, the result of pressure on 

 the extremities, seems to me probable. 



