444 Dr. I. A. Dixey on Mr. Merrifield’s Experiments 
ex hypothest diverse from those normal to the species, 
should favour one or other set of ancestral determinants 
at the expense of those more proper to the species. This 
would explain why the effect of heat differs from that of 
cold, though both lead to reversion. 
bere is, however, one fact which shows that the above 
explanation is not entirely adequate—the fact, namely, of 
the hereditary transmissibility of certain temperature 
modifications, as determined in the case of Polyommatus 
phleas by Weismann himself (‘f The Germ-Plasm,” 1893, 
p. 399). This phenomenon admits of a ready explanation 
under the theory of pangenesis ; the point that pangenesis 
fails to explain is the reversionary character of the original 
change, unless, indeed, we suppose a “struggle of gem- 
mules,” analogous to the “ struggle of determinants,” and 
continued, like the latter, throughout the ontogeny; in 
which struggle certain conditions favour the ancestral 
rather than the modern gemmules. But just as the 
theory of pangenesis seems to require some such addition 
as that suggested, so also, under the rival hypothesis, it 
seems necessary to supplement the explanation above 
given with another supposition already propounded by 
Weismann, namely, that the temperature-conditions are 
capable, in some cases, of actually altering the consti- 
tution of unexhausted determinants wherever they occur, 
even in the germ plasm of the ovum itself. 
Tam myself inclined to think that, granting Weismann’s 
general theory of heredity, the more special cases of 
reversion are to be chiefly explained, as above, by tie 
critical influence of the temperature-conditions on the 
struggle of the determinants, rather than by an intrinsic 
effect on the determinants themselves. The latter may 
account for such cases as a general lightening or darken- 
ing of the ground-colour, as in Weismann’s P. phleas, 
which strictly speaking are not really but only acci- 
dentally reversionary ; it will not, however, account in 
my opinion for the special ancestral marks shown by 
Mr. Merrifield’s V. atalanta and V. i. 
The point is capable of verification. If it be true that 
there is a selective influence which is exerted upon the 
actual struggle of the determinants, that influence would 
find a different expression in the adult according to the 
particular stage in the ontogeny at which the influence 
was applied, as it would affect those determinants only 
