It is necessary, in regard to this subject, to remember that many lower animals and plants can reproduce or propagate by separating “buds,” or large bits of their bodies, built up of thousands of cells, and, therefore, not to be confused with the single egg-cell. The egg-cell is a cell specially prepared for fusion with a sperm-cell, necessitating—except in very rare instances—the union in the new individual or young of living material from two separate parental organisms, and, therefore, in many cases, from two widely separate lines of ancestry. A snippet, or bit cut from a begonia leaf, will produce a new individual plant; a bit cut or torn from a polyp will similarly give rise to a new individual: but the parthenogenetic egg is not to be confused with these masses of cells. It is a true egg-cell which might have been fertilised, and it is found in animals such as insects and crustaceans, which are more highly elaborated in structure than any which, like the polyps and zoophytes, multiply by buds and cuttings. XXXVI THE DEADLY PHYLLOXERA It was only after long and patient investigation that the various broods of the terrible Phylloxera which, between 1868 and 1888, destroyed half the vineyards of France, became known, their relations to one another determined, and the final cure for the devastation caused by them decided upon and put into practice. In all ordinary plant-lice or green-fly (aphides) at the end of the summer, the last parthenogenetic brood produces a generation of distinct males and females, which differ a good deal in appearance from the virginal broods of the spring and summer. Each female, after receiving sperm-cells from a male, lays a single egg, which consists of a fertilised egg-cell enclosed in an egg-shell. It is deposited in a safe place in a crack of the bark of a tree, or on the rootlets of some plant, and remains unchanged through the winter. In the spring from every such egg hatches a single female aphis, which feeds and increases in size. In a very short time (a week or so) this solitary female (Fig. 58) proceeds to produce, without male intervention, young which grow from true egg-cells which are not laid but remain inside her. The young are born or pass out of her as small six-legged insects. They feed and grow up, and in turn produce “parthenogenetically” and viviparously broods of young like themselves. The first female thus hatched from the winter egg is called a “foundress,” or “stock-mother,” because she starts a whole colony of young which, by virginal propagation of successive broods, may number many millions in a season. These are known as “virgin-mothers” (Fig. 57), and eventually their later generations always produce males and females, so that we distinguish, in the course of a year, four sets of aphides, starting from the egg, namely (1) the foundresses, (2) the numerous generations of virgin-mothers, (3) the males, and (4) the egg-laying females.