The evidence against reckless reproduction at last, however, proved overwhelming. With whatever hesitation, Nature finally decided, once and for all, that it was better, from every point of view, to produce a few superior beings than a vast number of inferior beings. For while the primary end of Nature may be said to be reproduction, there is a secondary end of scarcely less equal urgency, and that is evolution. In other words, while Nature seems to our human eyes to be seeking after quantity, she is also seeking, and with ever greater eagerness, after quality. Now the method of rapid and easy reproduction, it had become clear, not only failed of its own end, for the inferior creatures thus produced were unable to maintain their position in life, but it was distinctly unfavourable to any advance in quality. The method of sexual reproduction, which had existed in a germinal form more or less from the beginning, asserted itself ever more emphatically, and a method like that of parthenogenesis, or reproduction by the female unaided by the male (illustrated by the aphis), which had lingered on even beside sexual reproduction, absolutely died out in higher evolution. Now the fertilisation involved by the existence of two sexes is, as Weismann insisted, simply an arrangement which renders possible the intermingling of two different hereditary tendencies. The object of sex, that is to say, is by no means to aid reproduction, but rather to subordinate and check reproduction in order to evolve higher and more complex beings. Here we come to the great principle, which Herbert Spencer developed at length in his _Principles of Biology_, that, as he put it, Individuation and Genesis vary inversely, whence it followed that advancing evolution must be accompanied by declining fertility. Individuation, which means complexity of structure, has advanced, as Genesis, the unrestricted tendency to mere multiplication, has receded. This involves a diminished number of offspring, but an increased amount of time and care in the creation and breeding of each; it involves also that the reproductive life of the organism is shortened and more or less confined to special periods; it begins much later, it usually ends earlier, and even in its period of activity it tends to fall into cycles. Nature, we see, who, at the outset, had endowed her children so lavishly with the aptitude for multiplication, grown wiser now, expends her fertile imagination in devising preventive checks on reproduction for her children's use.