(d) In the course of the life-history of the germ-cells, in the processes of final maturation, and in the mingling of hereditary factors that is effected in fertilization, there are opportunities for new permutations and combinations. Here is the crowning ad vantage of sexual reproduction, that it favours, more than the asexual process, the emergence of new variations.
(e) Without falling into a teleological fallacy, we may look further ahead and recognize that sexual reproduction among ani mals leads to dimorphic and separate sexes, whence follow court ship and the dawn of the love of mates. Sexual reproduction has been a factor in evolution, as regards, for instance, the emotions, family life and speech. The first use of the voice was doubtless as a reproductive call. One does not, of course, account for origins by indicating the advantages accruing from the steps taken, but it is legitimate to point to consequences as well as origins.
Growth tends to occur when nutrition is in excess of what is required for everyday recupera tion. In unicellular organisms the limit of growth is in most cases quickly reached, explicable exceptions occurring in special cases, such as giant Foraminifera, where there is a large pseudopodia) surface. The frequent multiplication of unicellular organisms has its counterpart in the frequent cell-divisions that occur in the developing Metazoa and Metaphyte (multicellular organisms). But in most Metazoa there is a more or less definite limit of growth—the physiological optimum of size. Further growth is apt to be attended by the setting in of some detrimental instability, and it is after the limit of growth has been reached that reproduc tion usually occurs. Special explanations are needed for peculiar phenomena like precocious reproduction or paedogenesis, as in the liver-fluke, the gall-midge Miastor (in which there are larvae within larvae), and some Urodela, e.g., Amblystoma and oc casional newts. It should also be noted that some fishes and rep tiles seem to have no limit of growth, and have a very prolonged succession of breeding periods. When the nature of the organism allows of a very large surface in proportion to size, as in trees, there seems to be no definite limit of growth. On the whole, how
ever, the proposition stands that reproduction does not usually occur until the limit of growth has been reached.
Abundant nutrition favours asexual multiplication, but a check to the nutrition may bring about the separation of the buds. A simple illustration may be found in Hydra, where a bud often produces buds of its own. Eventually a check to nutrition occurs and the buds drop off ; and this may be followed by a phase of sexual reproduction. Similarly a planarian worm in good nutritive conditions may form asexually a chain of four ; if a check to nutri tion occurs, the links separate ; and sexual reproduction may set in. Vigorously growing fruit-trees are often root-pruned because the check to nutrition favours the reproductive activity of flower ing and fruiting. But if foliage and vegetative activity are desired, it may be useful to nip off the flower-buds. Other things equal, abundant nutrition favours asexual multiplication, but the forma tion rather than the separation of buds. On the other hand, a check to nutrition may act as a stimulus to sexual reproduction.
The rate of reproduction de pends (a) on the constitution of the individual organism, and (b) on its immediate environment and nutrition. It is high in green flies and rabbits, low in golden eagles and elephants. The actual rate of increase, which is much more difficult to estimate, when a periodic census is not readily practicable, depends on the wide and complex conditions of life which are summed up in the phrase "the struggle for existence." Organisms sometimes show an extra ordinary increase in numbers in favourable areas and seasons, wit ness plagues of voles or locusts; and in exceptional cases, where food continues abundant and checks continue to be slight, the increase may go on for many years, as with the rabbits in Australia or the potato beetles in North America. But in most of the cases known to-day the sudden floods of life soon cease. The increase meets checks of famine and weather and enemies, and a balance is automatically restored.