October 28, 1921] 



SCIENCE 



395 



of these hormones is determined by the idio- 

 syncrasies of the enzymes of the germ cells. 

 The hormones that determine our personality, 

 constitute the bridge that connects this person- 

 ality on the one hand, with the specific enzymes 

 packed away in the chromosomes of the germ 

 cells, on the other. You and I differ by virtue 

 of the difference of atomic structure and atomic 

 activity of the enzymes and hormones which 

 make up that part of the stream of life-yeast 

 which has got into and is activating our proto- 

 plasm and will activate that of the fertilized 

 egg that results from us and our consorts. 

 Thus each is what he is in his physique, in 

 his thoughts and in his reactions largely by 

 virtue of the peculiar properties of those ex- 

 traordinary activating substances, which are 

 specific for him and other members of his 

 family and race or biotype. The future of 

 human genetics lies largely in a study of these 

 activities, and the origin of differences or 

 mutations in them. 



The study of hiiman genetics leads into 

 numerous fields of the physiology of human 

 reproduction. Of these one of the most signifi- 

 cant is that of twin-production. This topic 

 has many aspects. As is well known twins are 

 of two types. Two-egg twins come from two 

 eggs simultaneously ovulated and one-egg 

 twins arise by a division into two embryos of 

 a single young embryo. The two children 

 which thus arise from one egg are often so 

 marvellously similar that they are called 

 " identical twins." Now these identical twins 

 give a measure of the relative importance of 

 heredity and environment, as Francis Galton 

 pointed out. It is, indeed, marvellous to see 

 how such twins, even though living far apart, 

 retain their initial resemblancy, experience at 

 almost exactly the same time similar dis- 

 ease and emotional disturbances. Even the 

 thoughts, as measured by the so-called " asso- 

 ciation " tests and the finger prints are mar- 

 vellously similar. The dissimilarity of envi- 

 ronment has had little effect on altering the 

 rhythm of development, which is controlled 

 by an internal mechanism. The two -egg twins 

 are merely ordinary brothers and sisters who 

 are born simultaneously and though the in- 



trauterine environment and that of early years 

 is as nearly identical as possible, yet they are 

 as dissimilar as brothers and sisters are apt 

 to be. 



Though human heredity is the leading 

 branch of eugenical research, yet it is only 

 one. A fascinating branch of the subject is 

 that of mate selection, including a study of 

 those external and internal conditions that 

 control in this phenomenon. While propin- 

 quity is often considered the all-sufficient basis 

 of mate selection, yet statistical research re- 

 veals such facts as these ; that there is a selec- 

 tion of mates of corresponding divergence from 

 the mean in stature; that red-haired persons 

 do not marry as frequently as expected on a 

 random basis; that persons of opposite tem- 

 peraments tend to marry with each other. 



Research on fecundity, especially the differ- 

 ing fecundity of peoples having dissimilar so- 

 cial values in the population has not received 

 the attention it deserves; still we know some- 

 thing of the fractions of sons and daughters 

 of college men and women and have some 

 facts available towards a study of fecundity 

 of the socially inadequate. Always, however, 

 it is not to be forgotten that it is the residuum 

 of surviving children of a marriage that counts 

 in the race and the children of the less so- 

 cially adequate strains are permitted a larger 

 selective death rate than are those of the more 

 efficient strains. That is one reason why from 

 the less developed strains, vigorous and effect- 

 ive progeny are occasionally arising; while 

 some lines of the more effective and prosper- 

 ous families end in weak and lethal descend- 

 ants. Modern surgery has done much to keep 

 alive weak and defective individuals, but little 

 to improve racial qualities. Selection and its 

 effects, including those of war, have been all 

 too little studied. 



But fecundity of stocks is only a part of 

 the problem in a country which, like ours, has 

 in a single year, added about as much to the 

 population by immigration as by birth. Prob- 

 ably never before in the world has such a 

 migration of all sorts of races in such num- 

 bers, over so great a distance, taken place. 

 Here in America we have watched the process 



