. Discovery. Science. DISCOVERY 199 Sex and its Determination—I By J. S. Huxley, Fellow of New College, Oxford Sex appears to be absent in one great group bf organ- isms, the Bacteria. There are also here ancj there a few species of plants which only reproduce isexually —the banana, for instance, never sets seed ; it is therefore clear that sex is not a necessary actompani- ment of life. Why, then, is it so widespread? The answer is given bj^ the well-known facts of Mendelian heredity. Through sexual reproduction, the factors in the chromosomes are at each generation shuffled and recombin
. Discovery. Science. DISCOVERY 199 Sex and its Determination—I By J. S. Huxley, Fellow of New College, Oxford Sex appears to be absent in one great group bf organ- isms, the Bacteria. There are also here ancj there a few species of plants which only reproduce isexually —the banana, for instance, never sets seed ; it is therefore clear that sex is not a necessary actompani- ment of life. Why, then, is it so widespread? The answer is given bj^ the well-known facts of Mendelian heredity. Through sexual reproduction, the factors in the chromosomes are at each generation shuffled and recombined in new arrangements ; and this provides the possibility of combining separate advantageous mutations in a single stock. If, for instance, a tall pea with green seed-coat is; crossed with a dwarf pea with yellow seed-coat, all combina- tions wiU occur in the second generation—tall yellow, tall green, dwarf yellow and dwarf green. If tallness and yellow colour happened to be more advantageous than dwarf size and green colour, then it is obvious that any race which possessed both these characters would be weU placed in the struggle for exBtence. If crossing were impossible, such a race could only arise if both the favourable mutations were to occur in one line. To put it in the most general terms, we may say that, if separate mutations arise in a species in a given time, then if sexual reproduction does not exist, the result will be x varieties ; but if it does exist, then by recombination 2' varieties are possible. If the number of mutations had been ten, the number of varieties would be 10 in the one event, 1,024 iii the other. The existence of sex thus obviously favours constructive change, and makes it possible for a species, if the conditions in which it finds itseU alter, to adapt itself to them much more rapidly. It has been supposed that sexual fusion of cells was accompanied by some mysterious rejuvenation, with- out which the race would die out. This, however, i
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