DISCOVERY 



199 



Sex and 

 its Determination — I 



By J. S. Huxley, M.A. 



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, is 

 becoming more and more doubtful. For one thing, 

 it has been found possible by special treatment to 

 keep various unicellular animals like the Slipper 

 Animalcule (Paramecium) reproducing by fission for 

 apparently indefinite periods without any sexual 

 process of conjugation occurring, although conjuga- 

 tion is a normal process in their life-history. The evil 

 effects of inbreeding were supposed to proceed from a 

 similar lack of fresh blood, from the absence of that 

 fusion of gametes from different stocks which nonnally 



' Readers are referred to previous articles on Heredity by 

 the same authoi in Vol. I, pp. 199 ff. and pp. 2},iff.^ot this 

 journal. — Ed. 



happens in sexual reproduction. Here again we are 

 now able to give a different and more satisfactory 

 explanation. In an ordinary animal or plant, muta- 

 tions are occurring all the time. Many of these are 

 unfavourable — they represent little accidents to the 

 factors, to the machinery out of and by which a normal 

 organism is built up. But most of such harmful 

 factors are recessive ; that is to say, they may be 

 carried by an individual which is also carrying the 

 dominant factor of the same pair, without any effects 

 being produced ; this is so, for instance, in the case 

 of taU and dwarf peas, which when crossed give 

 hvbrids containing the factors for both tallness and 

 dwarfness, and yet indistinguishable by inspection 

 from their tall parent. In a large cross-bred popula- 

 tion, it will be only rarely that individuals containing 



::o-:-' 



Gametes 



of Female* 



(ova> 



a Gametes 



•••of Male 



• (sperms) 



FertiUsatiou. 



:aI. 6 



Female. Male. 



Pio. I.— DIAGR.WI OF THE BEHAVIOUR OF CHROMOSOMES \T 



REDUCTION AND FERTII,IS.\TION IN AN ANIM.-U, WITH TWO 



X-CHROMOSOMES IN THE FEMALE, ONE IN THE MALE. 



Reproduced from " Menddism," by Prof. R. G. Punnett, F.RS., by permission 



of the Autlior and of Macmillan & Co., Ltd. 



two of these harmful recessive factors will be segre- 

 gated out. But if inbreeding is practised, a little 

 calculation will show that it will produce a number 

 of different stocks, each of them pure for the various 

 factors which were present in the original population. 

 As a result, the recessive factors will appear pure in 

 a number of these stocks, and wUl exert there any harm- 

 ful effect they may have ; and the general average of 

 the population, in vigour, health, size, and fertility, 

 wUl go down very considerably. But the poor types 

 can now be rejected by the breeders ; and the good 

 types which are left are known to be pure and to 

 possess no more harmful recessive factors. Thus, 

 when the good types are now crossed together, a stock 

 is produced which is as good in appearance as the 

 original, and has the further merit of not containing 

 harmful recessive factors and therefore not continually 

 producing a certain proportion of low-grade individuals. 

 Thus the immediate effect of inbreeding on a large 

 mixed population is bad ; but if it is judiciously 

 practised, it may be the best means of building up a 

 pure healthy stock. That it cannot be always and 



