in Temperature-Variation. 443 
ancestral type, which though usually dormant might 
under certain circumstances become active in the onto- 
genetic process, it would yet seem a legitimate conclusion 
from the hypothesis, that the introduction of any cause 
analogous to hybridization in its action on the developing 
organism must belong to a far earlier stage in the onto- 
geny than the beginning of the pupal condition ; it must 
belong, in fact, to the stage of fertilization of the ovum. 
There are, however, a few facts on record, such as the 
assumption of ancestral characters by an old hen (Darwin, 
« Animals and Plants under Domestication,” 1868, vol. ii., 
p. 54), and the appearance of an earlier vertebrate con- 
dition in limbs of Amphibia reproduced after amputation 
(Ibid., 11., p. 15), which seem in some respects analogous 
to the present instances, as being apparently cases in which 
a disturbance of normal conditions at a comparatively 
late ontogenetic stage has in some way led to reversion 
in the course of the individual growth. These cases are 
regarded by Darwin as not incompatible with pangenesis, 
though not fully explained by it. 
Tf, on the other hand, we postulate with Weismann the 
existence of “ids”? and ‘‘ determinants,’”? endowed with 
the nature and properties that he supposes, the instances 
that we are considering become more explicable. For 
according to this theory every feature in the structure of 
the individual organism is the result of a “ struggle of the 
ids” in ontogeny, the final character of each histological 
unit being fixed at the moment of the liberation of its 
proper determinants by the disintegration of the “ids.” 
The competition between the carriers of heredity, many of 
which must under the theory be ancestral in character, so 
far from being confined to the ovum, is being waged 
throughout the entire ontogeny, and is renewed at every 
successive stage of development. This being the case, it 
is to be expected that any external influence, such as 
temperature, on coming into force at any given stage, 
should be able to exert an effect upon the struggle pro- 
ceeding at that particular time between determinants 
which are just beginning to play their parts in the onto- 
geny, and should in consequence be able to modify pro 
tanto the resulting adult organism. It would be, more- 
over, natural to expect the different determinants to be 
affected by different temperatures, nor would it be sur- 
prising to find that temperature-conditions, which are 
