72 Dr. F. A. Dixey on the phylogenetic 
P. ajax (all many-brooded species) are interpreted by 
the former author in a like sense. ‘l'hese results, how- 
ever, are confined to the artificial production of the 
oldest of several still extant forms of the same poly- 
morphic species ; whereas Mr. Merrifield’s experiments 
with a monomorphic and not very variable insect seem 
to have revived forms of the species older than any now 
extant. 
The possibility that exposure to greater heat as well 
as cold may induce reversion is alleged by Weismann 
(‘Studies in Theory of Descent,’ ed. Meldola, 1882, 
vol. i., p. 87). But that features so revived should be 
entirely distinct in the two cases, although ancestral in 
both, is, so far as I am aware, a quite new and un- 
expected result. 
It would seem that unless the whole alteration in 
these cases is the direct consequence of temperature 
conditions, which is unlikely, we must admit the possi- 
bility that a greater or less degree of atavism may be 
induced by disturbing conditions; the point to which the 
species ‘‘throws back” being controlled by the nature of 
the disturbance. Mr. Merrifield’s experiments with the 
double-brooded S. illustraria (Trans. Ent. Soc. Lond., 
1890, p. 131, &c.) appear to indicate that the case of 
this latter is not completely parallel with that of A. 
levana, inasmuch as the ‘“‘summer” as well as the 
“winter” form is in S. dlustraria capable of being arti- 
ficially produced (though with difficulty), whereas in A. 
levana, as is well known, it is not possible to artificially 
produce the ‘‘summer”’ form. ‘This fact with regard to 
S. ulustraria may be due, as he suggests (ibid., p. 142), 
to the direct effect of temperature; or, as he has also 
pointed out to me, it may possibly indicate that the 
summer form of S. illustraria is itself a reversion to an 
ancestral condition. If this latter be the case, the sup- 
posed explanation of the different effects of heat and 
cold in the instance of V. atalanta would receive some 
confirmation, for we should then have another example 
in which the point to which reversion was directed could 
be to some extent controlled. 
On the whole, therefore, it seems to me just con- 
ceivable that earlier forms which were developed under 
certain natural temperature-conditions may sometimes 
be under similar conditions independently restored. 
