CREATION BY EVOLUTION
is formed from a slime that gradually hardens. In this group, as in many others, the male is considerably smaller than the female.
Still higher up in the scale of progress we find a solitary bee, which also burrows into the ground—in gravel paths or among grass—and also stores its cells with honey and pollen. Although these bees are in a sense solitary they live in colonies that consist of large numbers;
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a colony may comprise a thousand cells. The sexes differ very much in appearance and are not often found together. The bees of this group are of economic value, for they aid in the fertilization of fruit trees. The bees of one particular branch of this group construct for a number of families a common gallery, which ramifies about in the soil, and these bees thus perform a certain collective or social work (Fig. 12). But the task of constructing each cell and of providing food for the larvae is the work of one family and not the collective work of many bees. Another group of bees falls under the common name of leaf-cutting bees (Fig. 13) . This bee is more robust than the ordinary hive bee and has a broader head. It makes nests in hollows in stems, in wood, or in the soil. The cell is made of leaves or of parts of leaves or petals of roses and other plants, which are moulded into a thimble-like form that has a lid composed of a smaller round piece of leaf. The cells are placed end to end and not side by side, and the pieces of leaves are gummed together. The string of cells thus made rarely exceeds seven. When completed each cell is half
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