DISCOVERY 



201 



these would doubtless be very grave) to apply it to 

 mammals and to human beings. 



The original reason for the wide occurrence of sex 

 is to be sought in the greater plasticity it confers, 

 the greater power of varying in response to changed 

 conditions ; but once it was established it reacted 

 markedly upon the later history of life. The gametes 

 are primitively alike ; then a division of labour occurs, 

 and the male gamete or sperm takes on the function of 

 finding the female gamete or ovum, which is concerned 

 with storing up food-material for the future develop- 

 ment of the embryo ; then the individuals which produce 

 the different gametes become different in other ways. 



that our instincts and the emotions associated with 

 them are the driving force of our actions ; that the 

 most primitive instincts, such as those of fear and of 

 sex, are perennially active in us ; but that the human 

 mind possesses the power called by psychologists the 

 sublimation of instincts, whereby the instinct becomes 

 directed towards other objects — its driving force 

 harnessed to new, more exalted, and more spiritual 

 ends. So fear becomes the basis of reverence and 

 awe, the sexual instinct gives rise to the highest 

 sympathy, the most universal love. 



Sex is thus intertwined, inevitably and fundamen- 

 tally, with all our activities and with our very being. 



* * //\ T T A /\ T /\ 



FIG. 4.— PEDIGREE OF A FAJIILY IN WHICH HEMOPHILIA OCCURRED ; AFFECTED INDIVIDUALS ARE M.iJRKED IN BLACK. 



J = male, $ = female. Note that the affected individuals are all males : that they do not produce affected children. 



Reproduced from " Th£ Treasury of Human Inheritance," by kind permission of the Director of the Gallon Laboratory, University of London. 



the male generally more active, the female generally 

 more passive and concerned with the nourishment and 

 care of the young. As mind develops, new complica- 

 tions arise ; in the first place, the female requires to be 

 courted and stimulated, her emotions roused, before she 

 will yield to the male ; from this cause there have arisen 

 the elaborate and wonderful ceremonies and displays of 

 courting animals, associated often with special colours 

 and structures. Sometimes the result is grotesque, 

 as in the wattles and bare coloured skin of the cock 

 turkey, or the coloured posteriors of many male 

 monkeys ; more often the effects are striking, as in 

 the gleaming metallic patches on the legs of many 

 male spiders, displayed to best advantage during their 

 strange courtship dances ; or they are of real beauty, 

 like the song of the nightingale or the thrush, the 

 colours of the cock humming-bird, the plumes of the 

 egret, the train of the peacock. It may be indeed 

 said that the sense of beauty has mainly sprung out 

 of the relation of the sexes, and that the actual beauty 

 of animals, where it does not depend simply upon 

 regularity of form, or upon the sense of power or of 

 speed or of vitality produced on us by certain crea- 

 tures, is due originallv to the existence of individuals 

 of separate sexes with emotions which must be touched 

 before sexual union can be consummated. Finally, 

 in man himself, recent work in psychology has shown 



Man and woman differ from each other, not only in 

 body but in mind ; and such is our mental architec- 

 ture that there are few activities of life in which the 

 sex-instinct, however transformed and sublimated, 

 does not play some part. It thus becomes of the 

 greatest interest to discover the mechanism by which 

 sex is determined, and to find out whether by any 

 means we can bring it under our control. 



To do this it is necessary to revert once more to the 

 lower animals. In discussing heredity, we said that 

 the chromosomes of any species were present in pairs, 

 the members of each pair being similar. In a number 

 of species there is an exception to this rule. In 

 certain insects, for instance, while all the chromosomes 

 of the female can be arranged in pairs, those of the 

 male cannot. On closer examination, this is seen to 

 be due to the fact that the male has one less chromo- 

 some than the female, and that therefore it only 

 possesses one instead of two of one particular kind of 

 chromosome. This sort of chromosome has been 

 called the X-chromosome. When the female comes 

 to form eggs, ordinary reduction occurs ; the two 

 members of each pair separate from each other, and 

 all the eggs receive one X. In the male, however, the 

 X has no mate to pair with ; accordingly half the 

 sperms will contain an X, half wiU be without one. 

 If a sperm with an X fertilises an egg, the result will 



