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in any respect serviceable to the life of the individual. All 

 the successive steps must have resulted directly from certain 

 external conditions ; and each step, as it was gained, must 

 have been passed on to the next generation by hereditary 

 transmission, until, in their respective geographical habitats, 

 the lighter and darker coloured races became ultimately fixed. 

 Could there, he might say, be a much clearer case of the 

 validity of Lamarckian, or rather Buffonian, factors] 



Clear as the case might seem, it did not satisfy Weismann. 

 He took into account a further fact, viz. that this butterfly in 

 a certain part of its range is seasonally dimorphic, alternating 

 in successive broods between the pale northern and the dark 

 southern form. This state of things he found hard to reconcile 

 with the theory of transmitted somatogenic modification ; for 

 if, as he argued, the colour of the race were becoming 

 gradually darker by the inheritance of each step in the 

 process as it appeared in the individual, no light-coloured 

 brood could be produced from dark parents, and vice verso. 



There was still another feature in the case to be reckoned 

 with. Weismann's own experiments on Araschuia prorsa- 

 levana had taught him that the susceptibility to the influence 

 of temperature is not uniform throughout the whole course of 

 the life-history, but belongs in greatest measure to one 

 particular period — in the case of A. pi'orsa-levana, as he 

 thought, to the early part of the pupal stage. I need here 

 do no more than mention incidentally that this observation 

 has been abundantly confirmed and extended, with modifica- 

 tions, by other investigators, notably by Mr. Merrifield ; and, 

 I may add, by Mr. G. A. K. Marshall, whose results, like those 

 of Lt.-Col. N. Manders, are especially valuable as having 

 been obtained under the peculiar difficulties of work in the 

 tropics, and as bringing in the operation of another factor, 

 namely, that of moisture. 



There must, then, be some constituent in the composition 

 of the individual which undergoes some successive changes 

 during the period of the individual life-history, and which 

 at one or more stages of its ontogenetic development is capable 

 of being acted upon by external influences in such a way as 

 to produce an obvious alteration in the final condition. But 



