THE MOTH AND THE CANDLE 231 



tropical species of May-flies are known which, like the 

 glow-worms and fire-flies, produce light in their bodies. 

 The May-flies, especially the males, have unusually large 

 and prominent eyes, as is the case with phosphorescent fishes 

 and some other light-producing animals, and it appears 

 probable that in the now rare instances of self-luminous 

 May-flies, the sexes are attracted to one another by the 

 light they produce, as is the case in other luminous 

 insects. It seems probable that the ancestral May-flies, 

 of which many remarkable kinds have been discovered 

 in the fossilized condition in strata as far back in time 

 as those of the coal-measures, were all self-luminous, 

 and acquired an overpowering instinct of seeking the 

 light given out by other individuals as a necessary step 

 towards sexual congress. In the course of ages other 

 senses (probably smell and touch) have been called in 

 to bring the fluttering insects into association. The 

 power of producing light, being no longer needed, has 

 disappeared from all but two rare species. But the 

 urgent erotic instinct, the nervous mechanism, which 

 drove the ancient May-flies towards the dancing lights 

 of other May-flies, has remained unaltered in all the 

 living species of the group. It is a " disharmony " 

 which has not been of sufficient destructive importance 

 to be " cleared away " or suppressed by natural selection. 

 In pre-human times, nocturnal fires and lights were too 

 uncommon to cause much disaster to the May-flies. 

 But now that mankind sets up everywhere his nocturnal 

 flames and electric lamps, the previously unimportant 

 useless survival of an overpowering impulse to rush to 

 nocturnal lights, reveals itself as a serious and death- 

 dealing " disharmony." We must suppose, on this theory, 

 that the other insects, such as moths and certain flies 

 (by no means all insects), which also madly fly into 

 nocturnal lights to their own destruction, have 'had 



