Page:The Effect of External Influences upon Development.djvu/50

46 companies long before the formation of insect-states. It was therefore possible for natural selection to step in and favour a definite mode of feeding, and at the same time to give those daughter-larvae an advantage in which some of the ids had varied in the direction of producing workers. We cannot yet however follow out the details of this process; and it would at present be premature to inquire how the poorer nourishment ever began to bring the worker-ids in particular into activity. This cannot be understood at present any more than can the peculiarity in the tissues of roots which causes them to grow downwards under the influence of gravity and not upwards, as the stems do. We must for the present content ourselves with the belief in the possibility of such a mode of reaction being provided for by processes of selection, as we know no other origin for purposeful modifications.

Thus in the whole history of metamorphosis everything apparently brings us back to selection. I have frequently been charged with exaggerating the sphere of activity of natural selection and with putting the direct effects of extemal influences in the background. But I hope to have shown today in one instance that there is sometimes a tendency to go to the other extreme, and to make extemal influences responsible for a metamorphosis in which they can have had no part. In the case in point, selection must be responsible for all that has become altered in the workers,—for the stunting of the ovaries, the variation in the nutritive requirements during the life of both larva