Page:The Effect of External Influences upon Development.djvu/52

48 a single queen appears to be regularly wanting and to be replaced by so-called 'substitution females' in Termes lucifugus, this condition is secondary, and has only arisen from the other long after the differentiation of castes had taken place. To this day true queens of Termes lucifugus appear, but found no state, for they perish in the nuptial flight.

But I have lingered too long on the state-forming insects, and must now return to the point in connexion with which I first referred to them. I wished to show that when it is important to regulate different possibilities of development, nature makes use of external influences as stimuli. But it is not always an external stimulus that originates a change of form;—it may be an internal one. Thus in the case of several animals which exhibit alternation of generations there appear to be internal arrangements for the determination of the sequence of the various forms: I was at least unable to prove experimentally as regards the Daphnidae that temperature and the drying up of the water in which they lived caused these animals to reproduce sexually. The various species of these water-fleas are so organized that each produces a tolerably definite number of generations of females, and then only are males born. The number of these generations, moreover, is definitely regulated in the various species, so as to be most conducive to their maintenance. In those that live in large lakes the males only appear at the end of the summer, while in such as inhabit small pools, and are exposed to the danger of desiccation, early production of winter-eggs is necessary; and these, as