234 



DISCOVERY 



Further, sometimes it may be possible to alter these 

 conditions, so that some of the male characters begin 

 to appear in a female, and vice versa. 



We are all therefore double, as far as secondary 

 sex-characters go ; but only one set is generally allowed 

 to develop, while the other is kept latent. Sometimes, 

 however, it is not kept completely latent ; and so 

 come about the abnormal individuals with a mixture 

 of the psychology' of the two sexes, or even a reversal 

 of sex-impulses. They are a problem to the psycho- 

 analysts, and are often looked upon with great moral 

 reprobation by ordinary people. But, in many cases, 

 there can be no doubt that their condition is no fault 

 of their own, but due to some slightly irregular working 

 of part of the machinery of sex-determination. They 

 arc, in fact, what Metschnikoff called " disharmonies " 

 in biology. 



Finally, there remains the problem of controlling 

 sex-determination experimentally. The problem is 

 still very incompletely investigated. In frogs, there 

 is no doubt that delay in fertilisation causes preponder- 

 ance of males ; and in birds and moths, crosses between 

 different varieties or species often result in individuals 

 which ought, by their chromosomes, to be females, 

 being transformed, sometimes incompletely and some- 

 times completely, into individuals of the other sex. 



In general, we may say that it is becoming increas- 

 ingly probable that, while the mechanism of the sex- 

 chromosomes provides the usual basis for sex-deter- 

 mination, yet other influences may sometimes modify 

 or override this mechanism, and that within the next 

 few decades we shall probably be able to control, in 

 some degree, the determination of sex in many species 

 of animals, and possibly even in man. 



(5) Heredity in M.\n ; and Rese.'\rch in 

 Heredity 

 What grounds are there for believing that inheritance 

 in man is based on the same arrangement of factors 

 and chromosomes, and follows the same rules and laws, 

 as in the lower animals ? The answer must be that 

 inheritance in man is of the same description as in the 

 rest of organised nature. The only essential differences 

 between man and the lower animals lie in his brain 

 and mental powers ; the rest of his physical basis is 

 in every way similar to theirs, and we should expect 

 it to have the same mode of inheritance. Further, 

 although naturally we cannot conduct on man, genera- 

 tion after generation, the elaborate breeding experi- 

 ments necessary to clear up the inheritance of a number 

 of characters, yet some human traits have been shown 

 to be transmitted in a Mendelian way. A type of 

 malformation, known as brachydactyly, for instance, 

 in which the fingers are abnormally stumpy, is inherited 

 as a simple Mendelian dominant ; while haemophilia, 



or the failure of the blood to clot properly, and some 

 forms of colour-blindness, are inherited precisely in 

 the way which we should expect for characters whose 

 factors are lodged in sex-chromosomes. 



But there remains the question. How far are mental 

 qualities inherited, and if inherited, is their inheritance 

 Mendelian ? To that it is as yet imp)Ossible to give 

 a definite answer. The researches of GaJton have 

 shown us that many mental aptitudes are definitely 

 inheritable, and if, as seems more and more certain, 

 mental characters are associated with the physical 

 characters of the cortex of the cerebral hemispheres, 

 there is an immense weight of probability for the view 

 that they arc determined in heredity in the same way 

 as any other characters of the organism. 



Mendelian analysis, however, is another matter. 

 Even with comparatively simple physical problems, 

 this may bristle with difficulties. For instance, size 

 in animals for many years evaded factorial explanation. 

 Only recently has Punnett shown us that, in fowls at 

 least, it appears to be due to Mendelian factors. The 

 analysis of mental qualities is incomparably more 

 difficult, partly because we cannot carry out definite 

 breeding experiments, partly because of the inherent 

 complexity of the characters involved, and partly 

 because actual mental structure is so plastic and so 

 largely determined by environment. 



Since mental qualities are the dominant ones for 

 humanity, it becomes clear that any legislation em- 

 bodying what may be called positive eugenics is not, 

 in the present state of our knowledge, practical politics. 



By negative eugenics, however, we might even now 

 accomplish much. There is a large body of evidence, 

 for example, that a certain type of feeble-mindedness 

 is inherited along Mendelian lines. If this is so, we 

 can do a great deal towards diminishing the suffering 

 and waste which it entails by preventing the repro- 

 duction of the feeble-minded, who, as a matter of fact, 

 at the present time, have a very high birth-rate. 



We have, however, reached a stage where eugenics 

 can and should become a real science. Before Galton's 

 time, it was the vision of isolated philosophers. During 

 the last fifty years, it has won for itself a position as 

 a pohtical idea to be reckoned with. The recent 

 development of genetical research has opened the way 

 for a scientific study of human heredity. When this 

 has been accomplished, we shall be ready for positive 

 eugenics, content not merely to raise the average of 

 society by cutting off the lower strata, but ambitious 

 to heighten the maximum of human attainments, and 

 definitely to assist the upward tendencies of evolution. 



(6) Research in Genetics 

 We have at last, through Mendelism, a real science 



of animal and plant breeding — the science of genetics. 



