3i8 



NA TURK 



[August 3, 1905 



LETTERS TO THE EDITOR. 



[The Editor does iwt hold himself responsible for opinions 

 expressed by his correspondents. Neither can he undertake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for this or any other part of Mature. 

 No notice is taken of anonymous commtmications.] 



The Problem of the Random Walk. 



This problem, proposed by Prof. Karl Pearson in the 

 current number of Nature, is the same as that of the 

 composition of n iso-periodic vibrations of unit ampli- 

 tude and of phases distributed at random, considered in 

 Phil. Mag., X., p. 73, 1880; xlvii., p. 246, i8qq ; ("Scien- 

 tific Papers," i., p. 491, iv., p. 370). If n be very great, 

 the probability sought is 



2 -'-V" , 

 -e rdr. 



Probably methods similar to those employed in the papers 

 referred to would avail for the development of an approxi- 

 mate expression applicable when n is only moderately 

 great. Ravleigii. 



Teriing Place, July 29. 



The Causation of Variations. 



It is sometimes said that natural selection has ceased 

 as regards civilised man ; but very clearly this is an error. 

 All civilised and most savage races are very stringently 

 selected by various forms of zymotic disease. Thus in 

 England practically everyone is brought into contact with 

 the organisms which give rise to tuberculosis, measles, 

 and whooping-cough ; those individuals who are the most 

 resistant to the organisms repel infection {i.e. do not fall 

 Ul), the less resistant suffer illness but survive, the least 

 resistant perish. Abroad, malaria, dysentery, and many 

 other complaints play a similar role. Probably no one is 

 absolutely immune to any disease ; but since illness only 

 follows invasion of the tissues by a sufficient number of 

 the microbes (the sufficiency of the number varying with 

 the individual attacked), and since the microbes are more 

 abundant in some localities than in others, the stringency 

 of selection as regards any disease is greater in some 

 places than elsewhere. For example, selection by tuber- 

 culosis is more stringent in the slums of cities than in the 

 country. It should^be noted, also, that resisting power 

 against any one '.disease does not imply resisting power 

 against any other ; thus an individual innately strong 

 against measles is not necessarily strong against tuber- 

 culosis. The result of all this elimination by diseases 

 demonstrates the action of natural selection very beauti- 

 fully. Every race is resistant to every disease strictly in 

 proportion to its past experience of it. Thus English- 

 men who have suffered much from tuberculosis are more 

 resistant to it than West African Negroes who have 

 suffered less, and much more resistant than Polynesians who 

 have had no previous experience of it ; that is, as a rule. 

 Englishmen, under given conditions, contract the disease 

 less readily, or if infected recover more frequently, or if 

 they perish do so after a more prolonged resistance than 

 Negroes and Polynesians. Negroes, on the other hand, 

 as South American plantation experience proves, are more 

 resistant to malaria than Asiatic coolies, who in turn are 

 more resistant than Englishmen and Polynesians. 



Against sorrie disea.ses {e.g. tuberculosis) no immunity 

 can be acquired, that is,- experience of the disease 

 confers no increase of resisting power,, the disease 

 pursuing a course of' indefinite length. Against other 

 diseases {e.g. measles) immunity may be acquired, that 

 i6,'Te.xf>erience of the disease, if not fatal, confers after a 

 definite time a more or less 'permanent immunity on the 

 sufferer." In the former case the survivors are mainly 

 those who have an inborn' power of resisting infection; in 

 the latter they are thosd who have an inborn pow'er of 

 recovering from infection; Evolution' has proceeded on 

 these lines. Thus Englishmen are less- readily infected 

 with tuberculosis than Polynesians, but nearly _all English- 

 men, like Polynesians, readily take measieSj . though a 

 much greater proportion of them survive and acquire 



NO. 1866, VOL. 72] 



immunity. Lastly, in relation to such very " mild " 

 diseases as chicken-pox, which render, the individual very 

 ill while they last, but cause hardly any elimination, . no 

 race appears to have undergone any change; for instance, 

 no race, apparently, is more resistant to chicken-po.x than 

 any other race. 



The pathogenetic organisms of all prevalent human 

 diseases are more or less entirely parasitic on man. Most 

 of them, therefore, flourish best in crowded populations, 

 where Ihey can pass readily from one susceptible individual 

 to another. Thus tuberculosis is most prevalent in the 

 slums of great cities. An important exception is malaria, 

 the parasites of which require special conditions, and 

 which, therefore, is more prevalent in the open country 

 than in towns. The inhabitants of the eastern hemi- 

 sphere have been afflicted by a multitude of zyn^otic 

 diseases for thousands of years. Of old, with the increase 

 of population, the conditions slowly laecame worse, the 

 stringency of selection became greater, and the human 

 races underwent continual evolution. But before the 

 voyage of Columbus zymotic disease, with the exception of 

 malaria, appears to have been almost, if not quite, un- 

 known in the New World. We have fairly definite 

 accounts of the first introduction of rnost . Old World 

 diseases to this and that aboriginal race, and of the 

 frightful destruction of life that followed, the principal 

 agent of elimination being tuberculosis. With their 

 diseases the European immigrants introduced modern 

 civilised conditions of life, especially churches, schools, and 

 other enclosed spaces in which the natives, crowded 

 together, conveyed infection to one another, arid clbthe_^, 

 which acted as a deterrent to cleanliness, and whitn, 

 besides, harboured the microbes of disease better than the 

 naked skin. As a consequence, except when protected 

 by malaria in extensive forests or when dwelling remote 

 in unsettled regions, the natives rapidly perished. It 

 is a significant fact that, whereas in Asia and 

 Africa every town inhabited by Europeans has its 

 native quarter, no European town in the temperate 

 parts of the western hemisphere {i.e. where tuberculosis 

 is most rife) has its native quarter. Published 

 health statistics demonstrate quite definitely that the 

 abnormally high mortality of the natives is caused by 

 introduced diseases. Since civilisation implies a dense and 

 settled population, it follows that no race can now achieve 

 civilisation that has not undergone evolution against 

 tuberculosis and kindred diseases. The case of the 

 Negroes is interesting. In Africa they had undergone 

 some evolution against tuberculosis. In America, when 

 they were first taken to it, the disease prevailed to a' com- 

 paratively slight extent, especially amongst the agricultural 

 population; but the conditions slowly became worse, and 

 the descendants of the early slaves underwent concurrent 

 evolution. To-day they are able to persist in the northern 

 cities, though their death-rate there is still abnormally 

 high. But though a constant stream of Negro slaves and 

 soldiers {e.g. in Ceylon) was poured for centuries Into 

 parts of Europe and Africa, they have left no trace on the 

 population. All perished in a few generations, the elimin- 

 ation being so stringent as to cause extinction, not evolu- 

 tion. It is tolerably certain that a fresh immigration of 

 African Negroes to America would end as disastrously. 



These facts appear to establish conclusively two truths, 

 first that evolution is due solely to natural selection, and 

 second that variations, except, perhaps, in rare Instances, 

 are not due to the direct action of the environment on the 

 germ-plasm, but are " spontaneous." The Laqiarckian 

 doctrine is quite out of court. If ever acquirerpents are 

 transmitted, it should be in the case of the profound and 

 lasting changes affecting the whole body which result from 

 disease ; but in no instance is the effect produced by any 

 disease on the race similar to that produced by it on the 

 individual. Thus tuberculosis injures the individual but 

 confers resisting power on the race ; measles confers 

 immunity on .the individual, but none on the race. Were 

 the Lamarckian doctrine, true, man could not persist on 

 the earth. Presumably, this is true of all other species, 

 since probably all , organisms are. subjected to causes of 

 slow deterioration similar to disease. If ever external 

 agencies acting directly on the germ-plasm .alter its com- 

 position and so cause variations (of any sort) in offspring. 



