Most of them place their cells vertically; but a few prefer the horizontal position, while still others, undecided as to the matter of direction, make clusters in which some are horizontal and others upright. Occasionally there is a remarkable innovation in building–material, as where in a group of fifteen, four cells in the centre were constructed of pure white plaster, forming a striking contrast to the surrounding mud color. One wasp built an entire cluster after an original fashion, following the beaten track until the cell was completed, and even bringing mud enough to daub it over, as her sisters were doing, but sticking it all in one spot, so that when the group was complete irregular lumps were attached here and there, leaving visible the elegant architecture of the individual cells. Did she think they were too pretty to spoil? or was she merely one of those radical spirits that rebel against conventionality and demand change for the sake of change? It is these variations that furnish Natural Selection with its materials; but rigid as may be the rules regarding the non–survival of the unfit, we find that the race of Pelopæus still produces many absent–minded wasps, that after spending hours in carefully constructing their nests, seal them up empty, forgetting to put in the spiders or to lay the egg. When a cell is sealed, the mother wasp ceases to take an interest in it, but she has done all that is necessary. In two or three days the egg hatches, after which the larva spends ten or fifteen days in eating, and then spins its cocoon. Here it remains, perhaps for only a few weeks,—for there are two or three generations in one season,—or perhaps through the long months of winter.