( Iv ) 



Mr. Tutt's assertion that melanism does not habitually occur 

 unless lower temperature is accompanied by increased hu- 

 midity ; qualifying the acceptance only by suggesting that 

 anything which would have the same effect as increased 

 humidity in diminishing the action of sunlight will probably 

 be found to produce the same results. 



There are many local circumstances which cause an inter- 

 ference with direct sunshine ; dense forests occur where the 

 full rays of the sun never penetrate, clouds and mist accu- 

 mulate around the summits of high mountains, fog and 

 smoke envelop the districts immediately surrounding our 

 manufacturing towns, islands in a temperate climate are 

 subject to condensation of moisture and sea-fogs, and under 

 all these circumstances dark varieties of certain species are 

 known to occur, although the same species when found under 

 different conditions more favourable to the action of light are 

 usually less intensely coloured. But if moisture is to be 

 taken as a direct, rather than an indirect, cause, we should 

 expect to find melanic variation occurring in the swamps of 

 Tropical Afi'ica, in the forests of the Amazons, on the banks 

 of the Mississippi, and in many other damp climates, even 

 within tropical regions, and I am not aware that this is the 

 case. 



It has been pointed out that no variation in the direction 

 of melanism has been found to occur in dry open level tracts, 

 however far north these may be ; and this fact, although it 

 tends to show that cold is not by itself a potent cause of such 

 variation, is perfectly consistent with the theory that 

 diminished sunlight exercises a certain influence upon the 

 direction in which colour may be expected to, and does, vary. 



It cannot be too freely admitted that in all cases of sup- 

 posed natural selection, accompanied by advantage to the 

 species, such advantage is probably by no means the sole and 

 exclusive cause of, or inducement to selection — all the special 

 conditions under which the species exists must be taken into 

 consideration, and any inclination to overrate the active 

 value of one special condition should be carefully discounted. 

 The study of such supposed causes and effects is yet in its 

 infancy, and although the promising child has "grown apace" 



