146 



THE entomologist's RECORD. 



DiAo. '2. 



colour) will have an advantage if the species rests on trees, 

 rocks, etc., No. 4 a greater advantage than No. 3, and so on ; 

 and vice versa in a dry season {vide Diag. i). 



" Now let us suppose the same species to vary, independently 

 345 in its relation to dryness and damp, con- 

 stitutionally. Let us arrange it again dia- 

 grammatically {vide Diag. 2), in order thus : — 

 "Then the individuals of No. i thrive best 

 when dry weather pushes them on rapidly, 

 but die in wet. No. 5 gets on well and 

 strongly in damp weather, but dies in dry. 

 (These I mean as tendencies, of course). Now 

 if we superpose these diagrams {vide Diag. 3), we have a set i-i, 

 that is doubly favoured by hot weather, viz., is pale in colour 

 and suits the weather constitutionally ; another set, 5-5, is 

 doubly suitable to damp weather, viz., is dark in colour and 

 suits the weather constitutionally ; the occupants of the other 

 squares are in intermediate positions. In a variable climate, 

 like that of the British Isles, it is obvious that, in some 

 seasons, i-i would be highly favoured, in others, 5-5 ; 

 whilst the others, 1-5, 2-3, 3-2, 5-1, etc., would always be 

 at a relative disadvantage, and we should in course of time 

 have a race consisting of i-i and 5-5, freely crossed together. 

 In a series of dry seasons, i-i would predominate, after a 

 series of wet seasons, 5-5.^ Now what would the effect of 

 I-I and 5-5 crossing together be ? Chiefly, probably, they 

 would result in a race indistinguishable from ^-^, but they 

 would possess the potentiality of readily, under suitable con- 

 ditions, evolving i-i or 5-5, which y-}^, if bred true in an 

 uniform climate would not possess. Each individual is, 

 though to appearance "y-^^ really nothing of the sort, but a 

 combination of i-i and 5-5, capable by selection of being 

 easily sifted apart, and the pure race of, say, i-i, evolved in a 

 few generations, whereas it would require a long period to do 

 so from a pure race of 3-3. Now the question I ask is this : — 

 Will it result also that the i-i element and the 5-5 element in 

 each individual exist in a so far independent state that if 

 placed in circumstances favourable to i-i, it will predominate 

 in that individual, or, if in circumstances favourable to 5-5, 

 the 5-5 element will assert itself during the life of that indi- 



' I leave out of view, for the sake of simplicity, the fact tiiat 2-2, 3-3 and 4-4 

 would also largely survive ; 1-5, 5~*i ^i*^li to a large extent, 4-1, 1-4, 2-5 and 5-2 

 would disappear. — T.A.C. 



