MELANISM AND MELANOCHROISM. 147 



vidual ? Experiment shows that, as a matter of fact, some 

 such reaction does occur in certain species, the point is, has 

 the possibihty of such a reaction arisen in the manner indi- 

 cated ? 



" Now, would the action of these two tendencies developed 

 under corresponding variations of weather, tend to produce a 

 physiological correlation between them in course of time ? 



" Diagram 3 illustrates hypothetical race 12345 

 varying independently in colour, and con- 

 stitutionally with regard to climate as sug- 

 gested above. 



" Or let us go over the origin of damp 

 producing varieties in this way : — Suppose 

 we have a race of Lepidoptera that has 

 not yet been influenced in this way (as a "^'^'"'"diITT' 

 matter of fact, there is probably none that has not been 

 so influenced over and over again), and assume, as is no 

 doubt true more or less of all races of Lepidoptera, that it 

 possesses (i) a tendency to vary to lighter or darker, and 

 also (2) a totally independent tendency to vary in the 

 way of adaptability to a drier or moister climate ; — then a 

 dry climate or season will preserve pale vars. and those 

 suited to dry conditions, and a wet season or climate 

 the dark vars. and those suited to wet conditions ; and 

 a fortiori those individuals will have the best chance that 

 combine suitability to climate in constitution and colour, 

 and these will appear more numerously in the progeny by 

 the crossing of those suited by constitution and those suited 

 by colour, till at length these two variabilities will become 

 correlated ; the further step that the wet (or dry) season 

 (or climate) shall elicit the latent adaptability to it, and 

 with it the corresponding colouring, is not quite so logically 

 evident, but I think it follows. We want really to know 

 whether exposure to wet (or cold) makes dark vars. or only 

 selects them. I do not know that the evidence of its making 

 them is yet very cogent ; but I think, if two kinds or sorts of 

 variability are thus made to occur together for countless 

 generations, such an effect might result. But it may be merely 

 that in a wet season dark specimens survive and are found — 

 in a dry season pale ones do so. I think my black DiurncBa 

 fagella resulted from a succession of seasons, not from one 

 season. The following diagram may make my idea more in- 

 telligible : — 



