MELANISM AND MELANOCHROISM. 
147 
Diag. 3. 
vidual ? Experiment shows that, as a matter of fact, some 
such reaction does occur in certain species, the point is, has 
the possibility 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 
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 5 
not yet been influenced in this way (as a 
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 
d 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 makmg 
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 Diuryicea 
fagella resulted from a succession of seasons, not from one 
season. The following diagram may make my idea more in- 
telligible : — 
