The conclusion seems to me inevitable, that the origination of the _Prorsa_ form was gradual--that those changes which originated in the chemistry of the pupal stage, and led finally to the _Prorsa_ type, occurred very gradually, at first perhaps remaining completely latent throughout a series of generations, then very slight changes of marking appearing, and finally, after a long period of time, the complete _Prorsa_ type was produced. It appears to me that the quoted results of the experiments are not only easily explained on the view of the _gradual_ action of climate, but that this view is the only one admissible. The action of climate is best comparable with the so-called cumulative effect of certain drugs on the human body; the first small dose produces scarcely any perceptible change, but if often repeated the effect becomes cumulative, and poisoning occurs.
This view of the action of climate is not at all new, most zoologists having thus represented it; only the formal proof of this action is new, and the facts investigated appear to me of special importance as furnishing this proof. I shall again return to this view in considering climatic varieties, and it will then appear that also the nature of the transformation itself confirms the slow operation of climate.
During the transition from the glacial period to the present climate _A. Levana_ thus gradually changed from a monogoneutic to a digoneutic species, and at the same time became gradually more distinctly dimorphic, this character originating only through the alteration of the summer brood, the primary colouring and marking of the species being retained unchanged by the winter brood. As the summer became longer a third generation could be interpolated--the species became polygoneutic; and in this manner two summer generations alternated with one winter generation.
We have now to inquire whether facts are in complete accordance with this theory--whether they are never at variance with it--and whether they can all be explained by it. I will at once state in anticipation, that this is the case to the fullest extent.
In the first place, the theory readily explains why the summer but not the winter generations are capable of being transformed; the latter cannot possibly revert to the _Prorsa_ form, because this is much the younger. When, however, it happens that out of a hundred cases there occurs one in which a chrysalis of the winter generation, having been forced by warmth, undergoes transformation before the commencement of winter, and emerges in the summer form,[18] this is not in the least inexplicable. It cannot be atavism which determines the direction of the development; but we see from such a case that the changes in the first two generations have already produced a certain alteration in the third, which manifests itself in single cases under favourable conditions (the influence of warmth) by the assumption of the _Prorsa_ form; or, as it might be otherwise expressed, the _alternating_ heredity (of which we shall speak further), which implies the power of assuming the _Prorsa_ form, remains latent as a rule in the winter generation, but becomes _continuous_ in single individuals.