( xcvi ) 
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 1 
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 versa. 
There was still another featui’e in the case to be reckoned 
with. Weismann’s own experiments on Araschnia 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. lyt'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. Mei’rifield ; 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 diflSculties 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 
