external influence—serves sooner or later to start the development of one kind or of another, as well as to decide which kind it shall be. Various degrees of difference may exist amongst these forms; they may refer merely to the colouration and marking—as in cases of the adaptive seasonal dimorphism which I have attempted to show probably occurs in caterpillars and butterflies,—or they may be so considerable as those which exist between the castes of state-forming insects.
External influences are in none of these cases the actual cause of the differences; they merely play the part of the stimulus that decides which of the primary constituents shall undergo development. The actual cause of these individual dissimilarities is in all cases to be sought in the preformed modifications occurring amongst the primary constituents of the organism itself; and such modifications, as they are always purposeful, can only have originated by selection. Even when to all appearance external influences have had a direct action in causing purposeful modification, a more careful examination of the case in point will always show that in reality they have only served to incite into activity some preformed adaptation. This is shown in a specially conclusive manner by the consideration of sterility in the workers of bees and ants. The sterility is not due to poor nourishment, but to the presence in the egg, owing to selection, of the primary constituents of a rudimentary ovary as well as those of a perfect one, the former undergoing development when the young larva is less richly nourished. Poorer food serves as the impulse which starts their development.
