August 2, 1906] 



NATURE 



531 



ii good many other naturalists, as indicating llie way in 

 which new species arise in Nature. The suggestion is a 

 valuable one if not very novel, but a great deal of observ- 

 ation will have to be made before it can be admitted as 

 really having a wide bearing upon the origin of species. 

 The same is true of those interesting observations which 

 were first made by Mendel, and have been resuscitated 

 and extended with great labour and ingenuity by recent 

 workers, especially in this country by Bateson and his 

 pupils. If it should prove to be true that varieties when 

 crossed do not, in the course of eventual inter-breeding, 

 produce intermediate forms as hybrids, but that characters 

 are either dominant or recessive, and that breeds result 

 having pure unmixed characters — we should, in proportion 

 as the Mendelian law is shown to apply to all tissues and 

 organs and to a majority of organisms, have before us a 

 very important and determining principle in all that relates 

 to heredity and variation. It remains, however, to be 

 shown how far the Mendelian phenomenon is general. 

 And it is, of course, admitted on all sides that, even were 

 the Mendelian phenomenon general and raised to the rank 

 of a law of heredity, it would not be subversive of Mr. 

 Darwin's generalisations, but probably tend to the more 

 ready application of them to the explanation of many 

 difficult cases of the structure and distribution of 

 organisms. 



Two general principles which Mr. Darwin fully recog- 

 nised appear to me to deserve more consideration and more 

 general application to the history of species than he had 

 time to give to them, or than his followers have accorded 

 to them. The first is the great principle of " correlation 

 of variation," from which it follows that, whilst natural 

 selection may be favouring some small and obscure change 

 in an unseen group of cells — such as digestive, pigmentary 

 or nervous cells, and that change a change of selective 

 value — there may be, indeed often is, as we know, a corre- 

 lated or accompanying change in a physiologically related 

 part of far greater magnitude and prominence to the eye 

 of the human onlooker. This accompanying or correlated 

 character has no selective value, is not an adaptation — is, 

 in fact, a necessary but useless by-product. A list of a 

 few cases of this kind was given by Darwin, but it is 

 most desirable that more should be established. For they 

 enable us to understand how it is that specific characters, 

 those seen and noted on the surface by systematists, are 

 not in most cases adaptations of selective value. They 

 also open a wide vista of incipient and useless develop- 

 ments which may suddenly, in their turn, be seized upon 

 by ever-watchful natural selection and raised to a high 

 pitch of growth and function. 



The second, somewhat but by no means altogether 

 neglected, principle is that a good deal of the important 

 variation in both plants and animals is not the variation 

 of a minute part or confined to one organ, but has really 

 an inner physiological basis, and may be a variation of a 

 whole organic system or of a whole tissue expressing itself 

 at several points and in several shapes. In fact, we should 

 perhaps more generally conceive of variation as not so 

 much the accomplishment and presentation of one little 

 mark or dilTerence in weight, length, or colour, as the 

 expression of a tendency to vary in a given tissue or organ 

 in a particular way. Thus we are prepared for the rapid 

 extension and dominance of the variation if once it is 

 favoured by selective breeding. It seems to me that such 

 cases as the complete disappearance of scales from the 

 integument of some osseous fishes, or the possible reten- 

 tion of three or four scales out of some hundreds present 

 in nearly allied forms, favour this mode of conceiving of 

 variation. So also does the marked tendency to produce 

 membranous expansions of the integument in the bats, 

 not only between the digits and from the axilla, but from 

 the ears and different regions of the face. Of course, the 

 alternative hairy or smooth condition of the integuments 

 both in plants and animals is a familiar instance in which 

 a tendency extending over a large area is recognised as 

 that which constitutes the variation. In smooth or hairy 

 varieties we do not postulate an individual development of 

 hairs subjected one by one to selection and survival or 

 repression. 



Disease. — The study of the physiology of unhealthy, 



NO. 19 18, VOL. 74] 



injured, or diseased organisms is called pathology. It 

 necessarily has an immense area of observation and is of 

 transcending interest to mankind, who do not accept their 

 diseases unresistingly and die as animals do, so purifying 

 their race, but incessantly combat and fight disease, pro- 

 ducing new and terrible forms of it by their wilful inter- 

 ference with the earlier rule of Nature. 



Our knowledge of disease has been enormously advanced 

 in the last quarter of a century, and in an important degree 

 our power of arresting it, by two great lines of study going 

 on side by side and originated, not by medical men nor 

 physiologists in the narrow technical sense, but by 

 naturalists, a botanist, and a zoologist. Ferdinand Cohn, 

 I Professor of Botany in Breslau, by his own researches and 

 by personal training in his laboratory, gave to Robert 

 Koch the start on his distinguished career as a bacteri- 

 ologist. It is to Metschnikoff the zoologist and embry- 

 ologist that we owe the doctrine of phagocytosis and the 

 consequent theory of immunity now so widely accepted. 



We must not forget that in this same period much of 

 the immortal work of Pasteur on hydrophobia, of Bohring 

 and Roux on diphtheria, and of Elirlich and many others 

 to whom the eternal gratitude of mankind is due, has 

 been going on. It is only some fifteen years since Calmette 

 showed that if cobra poison were introduced into the 

 blood of a horse in less quantity than would cause death, 

 the horse would tolerate with little disturbance after ten 

 days a full dose, and then day after day an increasing 

 dose, until the horse without any inconvenience received 

 an injection of cobra poison large enough to kill thirty 

 horses of its size. Some of the horse's blood being now 

 withdrawn was found to contain a very active antidote to 

 cobra poison— what is called an antitoxin. The procedure 

 and preparation of the antitoxin is practically the same 

 as that previously adopted by Behring in the preparation 

 of the antitoxin of diphtheria poison. Animals treated 

 with injections of these antitoxins are immune to the 

 poison itself when subsequently injected with it, or, if 

 already suffering from the poison (as, for instance, by 

 snake-bite), are readily shown by experiment to be rapidly 

 cured by the injection of the appropriate antitoxin. This 

 is, as all will admit, an intensely interesting bit of biology. 

 The explanation of the formation of the antitoxin in the 

 blood and its mode of antagonising the poison is not easy. 

 It seems that the antitoxin is undoubtedly formed from the 

 corresponding toxin or poison, and that the antagonism 

 can be best understood as a chemical reaction by which 

 the complex molecule of the poison is upset, or effectively 

 modified. 



The remarkable development of Metschnikofif's doctrme 

 of phagocytosis during the past quarter of a century is 

 certainly one of the characteristic features of the activity 

 of biological science in that period. At first ridiculed as 

 " Metschnikofifism," it has now won the support of its 

 former adversaries. 



For a long time the ideal of hygienists has been to pre- 

 serve man from all contact with the germs of infection, to 

 dcstrov them and destroy the animals conveying them, such 

 as rat's, mosquitoes and other flies. But it has now been 

 borne in upon us that, useful as such attempts are, and 

 great as is the improvement in human conditions which can 

 thus be effected, yet we cannot hope for any really com- 

 plete or satisfactory realisation of (he ideal of escape from 

 contact with infective germs. The task is beyond human 

 powers. The conviction has now been arrived at that, 

 whilst we must take every precaution to diminish infection, 

 yet our ultimate safety must come from within — namely, 

 from the activity, the' trained, stimulated, and carefully 

 guarded activitv, of those wonderful colourless amoeba-like 

 corpuscles whose use was so long unrecognised, but has now 

 been made clear bv the patiently continued experiments 

 and arguments of Metschnikoff, who has named them 

 " phagocvtes." The doctrine of the activity and immense 

 importance of these corpuscles of the living body which 

 form part of the all-pervading connective tissues and float 

 also in the blood, is in its nature and inception opposed to 

 what are called the "humoral" and "vitalistic" theories of 

 resistance to infection. Of this kind were the beliefs that 

 the liquids of the living body have an inherent and some- 

 what vague power of resisting infective germs, and even 



