. Discovery. Science. 200 inevitably bad is shown by the fact that there exist some animals and a number of plants in which self- fertUisation—the most rigid form of inbreeding pos- sible—is the invariable rule. In brief, it may be said that our understanding of Mendelism has made it clear that inbreeding is only bad when hidden harm- ful factors exist in the stock, and that it is harmless RED WHITE DISCOVERY Two quite distinct processes are involved in. the fertilisatiorj of egg by sperm which occurs in the ordinary sexual reproduction of man and most animals. First there is the fusion of two
. Discovery. Science. 200 inevitably bad is shown by the fact that there exist some animals and a number of plants in which self- fertUisation—the most rigid form of inbreeding pos- sible—is the invariable rule. In brief, it may be said that our understanding of Mendelism has made it clear that inbreeding is only bad when hidden harm- ful factors exist in the stock, and that it is harmless RED WHITE DISCOVERY Two quite distinct processes are involved in. the fertilisatiorj of egg by sperm which occurs in the ordinary sexual reproduction of man and most animals. First there is the fusion of two separate nuclei, two single sfts of chromosomes, and the consequent shuffling and recombination of the hereditary factors. In the second place, there is what is usually called aclivatioi—the starting-off of the egg upon its career of grow:h and development. In most species, if fertilisatbn does not take place, the egg remains inert, and sooaer or later perishes. However, in some animals, such as Aphids (plant-lice) and water-fleas, the egg is capable of developing without this stimulus. Such foms are called parthenogenetic. The bee is of especial interest, since the males, or drones, are father- less, produced parthenogeneticaUv, while the queens and wo'kers all arise from fertilised eggs. In the course of evolution, a change must have come about so that activation is no longer carried out by the sperm, but by some other means. What in these ^^i> Chrtmosomes («) of norual female Dro- sophila. Reproductd from " Mendelism,' Fig. 3. {b) Of normal male. (c) Of au XXY female individual. by Prof. R. G. Punnell, , by permission Fig. 2.—sex-linked inheritakce of white eye in DROSOPHILA. Females on left, males on right. The sex-chromosome behaviour is represented diagrammatically ; the chromosome bearing the factor for red eye is represented in black, that bearing the factor for white eye in outline. Reproduced from " ; by Prof. R. G. Punmlt,
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