January 20, 1910] 



NA TURE 



355 



:act, namely, that the disease is not communicable to 

 mimals, but only, so far as is known, to man. Hence 

 .xperimental studies on the disease could only be performed 

 on men who offered themselves voluntarily for this pur- 

 pose. Such experiments were sometimes negative, some- 

 times positive, in their result ; in the latter case, of course, 

 the subject of the experiment acquired the disease, and 

 in one case, at least, died of it. It would require the pen 

 of a .Shakespeare or a Milton to do adequate justice to 

 such devotion on the part of these brave men to the cause 

 of science and humanity. 



By numerous carefully devised experiments a number of 

 "important facts relating to the transmission of yellow fever 

 were elicited. It was shown that the unknown cause of 

 the disease is in the blood of the patient only during the 

 first three days of the illness, so that only during this 

 period can mosquitoes become infected by sucliing the blood 

 of the patient. Consequently, if the patient be protected 

 liom mosquitoes for the first three days he ceases to be 

 ;i danger to the community as a source from which the 

 infection can spread. It was shown, further, that the 

 mosquito, after acquiring the infection, goes through an 

 incubation period of from twelve to fourteen days, during 

 which it is not infectious : but after that it is infectious 

 for the rest dt its natural life ; and a further point of 

 interest was added by the French commission, namely, 

 that an infected mosquito may transmit the infection to 

 lis offspring, so that a mosquito whicli has never fed on 

 an infected person may be congenitally infectious. 



I have chosen the instance of yellow fever to put before 

 you because, although we have now such an accurate 

 knowledge, gained by experiment, of the cause and trans- 

 mission of the disease, no one has succeeded as yet in 

 s"eing the parasite itself. It is practically certain, for 

 inanv reasons, that there is some minute parasite at worlv, 

 and tliere are grounds for suspecting that the parasite is 

 a spirochaete, one of those minute, actively flexible, thread- 

 like organisms of which the affinities are so much in dis- 

 pute at present, and which some authorities class with 

 the Protozoa, others with the Bacteria ; but here we have 

 a case where the microscopist lias been baffled, and where 

 we get beyond the present limits of the powers of our 

 instrument, a fact which should make us appreciate the 

 labours of those who study the microscope and strive to 

 perfect it. 



Did time permit, I might mention many more important 

 discoveries in the field of protozoan parasites causing 

 disease. For example, there are tlie blood parasites of the 

 genus Piroplasma (Babesia), causing fatal forms of hemo- 

 globinuria in various animals ; they are not yet known for 

 certain in man, but a species is known from monkeys, a 

 source which is getting perilously near to us. Here the 

 agent of infection is a tick of some sort, and usually the 

 infection goes through two generations of ticks, being trans- 

 mitted from the mother-tick, which has acquired the in- 

 fection, to the numerous progeny of minute six-legged tick- 

 larve, which in their turn infect the vertebrate host. Then 

 tliere are the relapsing fevers caused by spirochetes in the 

 blood, and said to be transmitted in Europe by bed-bugs, 

 but in Africa by a species of tick which lives in mud floors. 

 In India and other parts of the tropics we find that the 

 deadly disease known as kala azar, due to a parasite, is 

 transmitted in all probability by bed-bugs. All these and 

 many others furnish points of great interest, but I must 

 be content with the three examples witli which I have dealt 

 in more detail, in order to show you how great a work 

 has been done and is being done in this field. As Prof. 

 Osier said recently, these discoveries are going to have an 

 f-normous influence on the history of the world and of 

 mankind, because they are going to make the tropics 

 habitable by white men. We hear or read so often of 

 such-and-such a country being uninhabitable by Europeans 

 ■an account of its deadly climate; but when we look into 

 the matter we find that it is not the climate at all that 

 is to blame, but that the white races are killed off by 

 vliseases caused by some animal parasite with which thev 

 nre inoculated by the bite of some bloodthirsty arthropod. 

 Take Uganda, for instance, with which I have a slight 

 acquaintance ; all that the climate does for you there is 

 to give you a sunstroke if you go out in the heat of the 

 <lay with inadequate headgear, and to make it very difficult 

 NO. 2099, VOL. 82] 



to keep awake after lunch. Some well-known European 

 diseases, such as small-pox and syphilis, are also rife 

 there ; but, on the other hand, some of our familiar 

 plagues, such as tuberculosis, rheumatic fever, and 

 influenza, appear to be absent. The diseases that are really 

 to be feared are all such as spring from bites of arthro- 

 pods. If you protect yourself from the mosquito you will 

 not get malaria ; avoid the tsetse-fly, which is very easily 

 done, and you are safe from sleeping sickness ; do not 

 sleep on mud floors, nor pitch your tent on old encamp- 

 ments, and relapsing fever will not trouble you ; keep rats 

 and fleas at a distance, and you are safe from plague. 

 With a little care and attention to surroundings the 

 European finds his life in the tropics, if anything, more 

 free from disease than in our temperate but influenza- 

 ridden Palsarctic climate. 



In the foregoing remarks I have directed attention more 

 particularly to the practical results of microscopy wedded 

 to sagacious experiment, and have tried to show how fertile 

 in good results this union has been, and promises stilly to 

 be ; but I would not have you go away with the impression 

 that I advocate such studies solely on account of immediate 

 practical good to be derived from them. Far from it. I 

 am one of those who hold so-called theoretical and un- 

 practical studies to be of the highest importance, and 

 worthy of all support, if only for the reason that, being 

 unremunerative, they often cannot support themselves. All 

 history shows us that the knowledge of general principles 

 must precede their application and practice, and that what 

 is purelv theoretical in one generation becomes thoroughly 

 practical in the next or in a later one. There is no need 

 for me to waste your time by multiplying instances of this 

 familiar truth : but I will conclude with a few words on 

 the wider applications of microscopy. 



In the range of the natural sciences, two branches of 

 knowledge stand at opposite poles, as judged from the 

 standpoint of the objects with which they deal. The 

 science of astronomy deals with the infinitely great; the 

 science of biology, on the other hand, with the infinitely 

 small. The astronomer with his telescope astounds us with 

 the distant worlds he reveals to us ; he thinks in millions 

 of miles as ordinary persons deal with feet or yards ; and 

 he exhibits to us this world on which we live as but an 

 insignificant planet, one of many, whirling round a star 

 far inferior in magnitude to many of those we see nightly, 

 a tiny speck in the vast ocean of space and matter, peopled 

 bv a race of puny creatures who style themselves the lords 

 of creation, although their dominion does not extend over 

 a billionth part of the universe. " The consciousness of 

 an endless series of worlds," said Kant, " destroys _ my 

 sense of importance, making me only one of the animal 

 creatures which must return its substance ag.ain to the 

 planet (that, too, being no more than a point in space) 

 from whence it came, after having been in some unknown 

 wav endowed with life for a brief space." 



Not less astounding, but in a totally different way, are 

 the revelations of the biologist with his principal _ instru- 

 ment of research, the microscope. With this he discovers 

 continually new worlds invisible to the unassisted eye, and 

 reveals infinite complexity in things apparently the most 

 simple. We find, in the first place, our own bodies to_ be 

 microcosms, small worlds, that is, of such inexhaustible 

 variety and elaboration of detail that to the human mind 

 Ihev are as difficult to comprehend and to realise in their 

 entirety as the macrocosm or great universe itself. We 

 find, further, that each huni.in" bodv, itself appearing as 

 a single individual or unit, is in reality made up of many 

 billions of living units or cells, each as much a microcosm 

 as the whole bodv ; and thus our instrument, the micro- 

 scope, brings us face to face with the greatest mystery 

 in the whole range of the sciences, namely, the problem 

 of life and living "matter. There is. apparently, no gap_ in 

 nature so profound as that which senarates the living 

 from the not-Iivina. The nature of life, its origin and 

 destiny, the laws that govern living matter and vital pro- 

 cesses of all kinds, these are of all problems not merely 

 the most fundamental in science and philosoohy, but also 

 the most important for our practical knowledge and daily 

 conduct. 



It would be futile to assert that human science has as 

 yet made any great advance in elucidating the na^Te of 



