Page:The Effect of External Influences upon Development.djvu/49

Rh it does not seem to me surprising that natural selection has in both these cases coupled the development of the caste primary constituents with stimuli dependent on nutrition. This convergence seems to be as intelligible as the independent development of similarly constructed eyes in very different groups of animals. What other influence could have acted as the stimulus, and at the same time been under the control of, and dependent on, the will of the animals?

It might be supposed that different degrees of temperature would have served this purpose equally well; but it would have been difficult to make such a stimulus effective.

Ants do at times carry their pupae into the sun; and it would doubtless have been possible for natural selection to connect the development of the primary constituents of the workers with stimuli produced by temperature, just as in the adaptive seasonal dimorphism of butterflies the different garbs of the members of the species might have been made sensitive to definite temperatures—provided it were possible for natural selection to couple the development of all the characters of the workers with the stimulus of temperature. But how would the necessary temperature have then been secured for the larvae, seeing that the animals have no control over the sunshine, and can still less command an ice-cellar?

It is however not diflicult to understand how differences in nutrition came to be a determining stimulus, for the feeding of the larvae was in vogue among the solitary Hymenoptera and such as lived in small