266 



Supplement to '' Nature^' August i8, 1923 



Stockard ^^ induced permanent changes in the germ- 

 plasm of guinea-pigs by exposing one generation of 

 animals to extreme and continuous doses of alcohol. 

 Dr. J. G. Adami '-" cites several instances of a similar 

 nature, and has summed up the evidence relating to 

 " the inlicritance of acquired conditions in the higher 

 mammals." Many of the cases recorded to prove 

 acquired inheritance relate to changes which have 

 been produced in the skin, particularly in its pigment- 

 carrying cells. On the evidence which has accumulated 

 there is good reason for believing that light can act 

 upon epidermal and other elements of the skin in such 

 a way as to effect changes in certain factors or elements 

 of the germ-plasm. The observations and experiments 

 made by J. T. Cunningham ^9 on the colouring of flat 

 fish, and the more recent observations which Dr. 

 Kammerer ^ has made on salamanders exposed to 

 light, and to dark backgrounds, can be interpreted 

 only if we admit that reactions in the skin can affect 

 the reproductive cells lying within the genital glands 

 of the animals subjected to experiment. Notwith- 

 standing this admission I do not think, as I shall 

 mention later, that the loss of pigment in fair Europeans 

 is due to any' direct action of light on the skin. It 

 is one thing to injure or influence the germ-plasm in 

 such a way as to alter the machinery which controls 

 the development of the embryo ; it is quite another 

 thing to alter that machinery in such a way as to make 

 it produce a new mechanical adaptation. We know 

 of no means by which the machinery of mechanical 

 adaptation can be altered from without. 



Are the Modern Conditions of Life altering 

 THE Germ-plasm of the Human Stock. 



The admission that the genital cells can be injured 

 or altered by substances circulating in the body of 

 the parent is of the utmost consequence for mankind. 

 The conditions of modern civilisation are making us 

 the subjects of a colossal experiment. Six thousand 

 years ago, our ancestors, scraping a subsistence from 

 moor and shore, passed their days amidst the same 

 conditions as surrounded the earliest types of evolving 

 man. Man's body was adapted for rough fare and 

 unregulated exposure. Modern civilisation has revo- 

 lutionised the conditions of life in every detail. We 

 use our brains, our skins, our muscles, our lungs, our 

 teeth, stomach, and bowels^ our hands and feet, for 

 purposes which are new to them. Our tissues are 

 kept soaked with juices containing substances which 

 are still strange to them. Our crowded communities 

 favour the prevalence and spread of all forms of 

 infectious disorders in young and old. We are dis- 

 covering that a rough and raw dietary contains certain 

 elements which are essential for health. It would be 

 strange if the evolutionary machinery of the human 

 body kept on working in the same way as when the 

 conditions of life were, if not simpler, yet much more 

 primitive. A prolonged and minute comparison of 

 human remains found in ancient and modern graves 

 in England has convinced me that structural changes 



" " An Experimental Study of Racial Degeneration in Animals treated 

 with Alcohol," Archiv Int. Med., 1912, vol. 10, p. 369 ; Proc. Soc. Experim. 

 Biol, and Med., N.Y., 1911-12, p. 71 ; 1913-14, p. 136. 



" Medical Contributions to the Study of Evolution, 1918, ch. v, 



" Hormones and Heredity, 192 1. 



" Nature, 1923, vol. m, p. 637. 



of a minor kind are affecting certain parts of the 

 skeleton in at least one-third of modern instances. 

 The narrow bony opening to the nose, with its jilMike i 

 nasal spine, its raised and sharp sill, so often seen in i 

 modern English skulls, are (onditions never present j 

 in l<2nglishmen of the pre-Koman periods. Contracted ' 

 palates, crowded and defective teeth, deformed jaws, 

 sunken cheek-bones do not become common in English 

 graves until we reach the eighteenth century*. The 

 appearance of these structural changes in Englishmen 

 cannot be attributed to the introduction of any new 

 racial element from abroad. No doubt these facial 

 changes are due in part to the soft nature of our food, 

 and the disuse of our muscles of mastication. 



Lack of use alone will not, however, explain thi- 

 form taken by these structural alterations ; they are 

 injurious rather than helpful ; they cannot be classified 

 among the contrived adaptations. We have rea.son 

 to suspect that defects of eyesight grow more common. 

 There arc grounds for believing that the great bowel, 

 including the caecum and appendix, becomes more 

 lial)le to disorder and to disease with each succeeding 

 generation. Twenty years ago Metchnikoff '^ expressed 

 the belief that the great bowel of man had become a 

 useless structure, and that he would be better off 

 without it. The result of recent surgical experience 

 has been to convince medical men that the man with 

 a normal great bowel is an infinitely fitter and happier 

 person than the man without one. The only question 

 that remains to be settled is whether it is better to be 

 with or without a colon which has become incurably 

 diseased. 



There is thus a certain amount of evidence to support 

 the belief that certain parts of the body are less robust, 

 some of them actually undergoing a structural change, 

 in a considerable proportion of people living under 

 modern conditions of hfe. There is also no doubt 

 that these changes and susceptibilities occur much 

 more frequently in some families than in others. To 

 what extent these new features have become hereditary 

 and therefore due to an injury of the germ-plasm, we 

 cannot yet say. But in the light of experiments like 

 those of Guyer and Smith, and of Stockard, medical 

 men have grounds for suspecting that the source from 

 which new generations of our race issue may not be 

 invulnerable, that our germ-plasm may become tainted 

 under the conditions to which our bodies arc now 

 subjected. 



The Law of R.ECAPiT«^.^^'^ON is o>rt\^TXltTiALLY ' 



TRUE;___ 



In the foregoing paragraphs I have turned aside 

 from my main thesis — the nature of the evolutionary 

 machinery which has given man his gifts of brain and 

 body. The nature of this machiner\- will ne\-er be 

 understood by those who still harbour the belief 

 that the human embryo, in its developmental stages, 

 recapitulates the evolutionary histor>' of the human 

 body. I do not think any one familiar with the stages 

 passed through by the developing human embryo 

 would now agree with Huxley when he wrote : ^ — 



" The Nature of Man, translated by Dr. P. Chalmers Mitchell, 1904 ; 

 see also Keith, " The Functional Nature of the C»cum and Appendix," 

 Brit. Med. Joum., 1913, vol. 2, p. i599- 



" Collected Essays, vol. 2, p. 5- 



/ 



