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its colour so as to be very difficult to detect ; this was known 

 to take place frequently among the Reptilia. 



Mr. A. E. Cook exhibited a variety of Smerinthus tihcB, L., 

 the lower part of the central band on the primaries being absent. 



Mr. J. Jenner Weir exhibited Mynndeon eiiropceus, L., bred 

 by him from larvae he obtained at Fontainbleau in 1887. 



Mr. J. Jenner Weir read an extract from a letter addressed 

 to him by Mr. Cockerell, dated 21st of May, 1888, refering to 

 Mr, Cockerell's theory that Go7iepteryx rhavini diXid Gonepteryx 

 Cleopatra originated as seasonal races, or as he should term it, 

 horeomorphic races ; — 



"(i) I take it that dimorphism in animals is something like 

 the case of certain salts which are dimorphic in the crystals;, 

 that is to say, as the salt may crystalize either as a cube or 

 octahedron (say), so the primitive germ may develope either 

 into one form or the other, and a very good instance of this is 

 the familiar case of male and female (in fact the sexes remain 

 undifferentiated till quite late in embryonic life). Now this 

 differs from simple variation, which is wholly the result of 

 circumstances ; it is doubtless originally a case of simple 

 variation, but when it has reached the stage we call 

 dimorphism, the effect of outside influences is like a small 

 weight in the balance, determining which tendency shall 

 prevail, and originating no new character. 



" (2) Therefore, the mere fact of two species flying together 

 is favourable, rather than otherwise, to the view that they 

 arose as dimorphic varieties ; because two species could not 

 have been produced under the same conditions by ' simple 

 variation,' unless by natural selection, which is improbable in 

 the present case. 



" (3) In certain districts of America Colias eurytheme and 

 Colias keewaydin fly together and at the same time, though 

 otherwise they are alternating seasonal races (vide W. H. 

 Edwards' Butterflies North American Colias, iv.) ; so much for 

 the two species flying together. 



" (4) Is it certain that GoTiepteryx cleopatra is never double- 

 brooded ? But if it is not, I do not see any difficulty in 

 supposing our originally double-brooded species becoming 

 single-brooded ; and to prove this possibility I will take the 

 same species that you have taken, viz., Pieris napi. 



