68 



KNOWLEDGE 



[Nov. 25, 1881. 



matter of doubt To sploct a ainglo illustration of the 

 application of tliis tliwry, wo nmy take tlif> casp of small- 

 pox, and its aniilogous condition, tlio fcvpr produced by 

 vaccination. When an infant is vaccinated, the physician 

 introduces into its system, thro\igh an abrasion of its skin, 

 a minute quantity of vaccine lymph, which, as every- 

 body knows, is obtained either from the vaccination 

 pustule of an already vaccinated child, or direct from 

 the calf. In either case, there are ijitroduced into the 

 infant body, certain minute germs — suspended in and 

 living naturally amongst the vaccine lymph — and in 

 due course these germs multiply and increase within the 

 frame, tln-roby proclucing the characteristic fever, and the 

 equally characteristic pustule at the seat of the operation. 

 So, also, with smalt-pox, which vaccination imitates in a 

 mild way, and of which, moreo\-er, it is a preventive. 

 Here the germs of small-pox, obtained directly or indirectly 

 from an already infected person, attack the body. Gaining 

 admittance thereto, they propagate themselves within the 

 tissues and through the medium of the blood. Sooner or 

 later all the characteristic symptoms of the disease arc 

 manifested, and having run its course, it dies away as 

 mysteriously, to all appearance, as it came. Now, there is 

 something strikingly analogous in all this to the growth of 

 an animal or plant There is a period of " incubation " in 

 the fever, just as in the production of the living being 

 there is a period of development. Tliere is a growth of 

 the fever, as the animal or plant grows towards its ma- 

 turity ; and there is a decline of the disease, as the living 

 form passes to its old age and death. So far, then, the 

 parallel between ordinary life and the birth, growth, and 

 decline of a disease, is very close and clear. 



But the analogies are not yet exhausted. Each fever 

 produces its like, as do animals and plants. Each disease 

 reproduces its kind, as Tyndall has somewhere observed,* as 

 rigorously as dog and cat reproduce their like. The pheno- 

 mena, or, as a doctor would call them, the " symptoms," of 

 each disease are, as a rule, highly distinctive. The 

 symptoms of scarlatina are not those of small-pox ; 

 measles is diflcrent from the other two ; whilst typhus 

 fever is again thoroughly different from all three. Analogy 

 may, as Darwin says, be a deceitful guide ; but when the 

 facts are so closely allied, as are the facts of epidemic 

 diseases to those of animal and plant development, the 

 use of analogy cannot be doubted in rendering the relation- 

 ship clearer. 



\Ve are now in a position to understand more clearly 

 the utility and strength of the germ theory in certain of 

 those aspects which bear most materially on science at 

 large. It would only serve to strengthen the idea that 

 our epidemic diseases are .simply the offspring of lower life, 

 if we reflect in passing that there are known to science a 

 very considerable number of lower plants which produce 

 in man's skin effects and diseases as characteristic as those 

 which a fever induces in his system at large. Thus, the 

 disease known as " ringworm " is caused by the growth in 

 the human skin of a parasitic fungus, and a whole series 

 of skin affections is known wherein lower plants play the 

 part of direct causes. Thus, if it is a matter of certainty 

 that a particular skin-disease is caused by lower plant- 

 growth, so no less is it by analogy likely that all other con- 

 tagious and epidemic diseases are in reality the products 

 of life. 



So much for the general idea that permeates the " germ- 

 theory " of diseas(\ Within the jiast few months some 

 highly important additions have been made to our know- 

 ledge of the part played by lower organisms in the produc- 



* Quoting a romnrk by Miss Xiglitinfralo. — Ed. 



tion of disease. M. Pasteur, whose researches into thf 

 development of lower organisms have placed him in the 

 foremost rank of scientitic workers, has detailed at length 

 the results of his investigations into the causes which i)ro- 

 duce the curious disease known a.s chnrhon, atUhrar, and 

 njilfnic ferer. This disease, whilst but rarely attacking 

 man, is fatal to horses, cattle, and sheep. France suffers 

 greatly from this " plague of boils," and it is also known 

 in various other countries as a literal scourge. Pasteur, it 

 should be mentioned, had already acquired much valuable 

 experience in the investigation into the cause of the 

 p'clirine, or silkwonn disease, which in 18C3 had devastated 

 the silk industry of France. Pa.steur showed that pihrine 

 was caused by the growth and multiplication, within the 

 bodies of the insects, of minute " corpuscles," which were 

 practically lower forms of life. Even the eggs from which 

 the worms were hatched were shown to bo liable to in- 

 fection from pchrin'' ; the eggs, in such a ca.se, inheriting 

 the disease from the parent moth which laid them. As 

 the result of a long and laboriovis series of experiments, 

 Pasteur showed that the pebrine would spread like an 

 infectious disease by the contact of whole with disea-sed 

 worms. He showed that, just as man isolates his fever 

 patients, so the French silk-grower had to isolate and 

 separate his diseased wonns. But the knowledge which 

 led to this effectual result was knowledge that had been 

 won by an u])hill fight, and that had been gainefl by the 

 object-glass of the microscope, and by the whole-souled 

 de\-otion of many months" industry. 



THE LAWS OF PROBABILITY. 



THERE are few subjects in which men take a more 

 general interest, yet few in which they make greater 

 mistakes, than the subject of probaliilities. From the 

 man whose mind is most perfectly trained in the analy.sis 

 of evidence, down to liim whose thinking apparatus can 

 scarcely be reg.arded as a mind at all, all men endeavour to 

 guide their conduct in matters uncertain, or at least to 

 form their opinion on such matters, according to the pro- 

 baliilities. They would use different words in describing 

 their purpose. A mathematician might, perhaps, speak 

 definitely of the a priori and ct posteriori probabilities in 

 favour of an opinion or of a course of conduct. Wendell 

 Holmes' " lout," he " who lies outstretched on a tavern 

 bench with just mental activity enough to keep his pipe 

 from going out," would hardly use the same expressions ; 

 but in his imperfect way he is all his life doing the same 

 thing. He does not even stretch out his ungainly limbs on 

 one liench rather than on another, or in one attitude rather 

 than another, without to some degree considering his chance 

 of comfort or security. But in all the ordinary relations 

 of life he more definitely weighs the chances, though often, 

 if not always, in an utterly inexact balance. 



It is not the loutish mind only, howe\'er, nor the average 

 intellect, which inex.actly estimates probabilities. The 

 most profound knowledge of the mathematics of chance 

 does not save men from eiTor. Often, indeed, the grossest 

 blunders have resulted from an attempt to weigh ordinary 

 matters in a mathematical balance. The fault has not lain, 

 however, with the balance, but with the user. If a man chooses 

 to weigh groceries in a chemical balance, paying no attention 

 to the fact that either the parcels ho weighs come in 

 contact with other things lying round, so that the balance 

 cannot possibly show their true weight, or else, perhaps, 

 th.at the construction of the balance is such as will only 

 penuit of its indicating true results within cei-tain very 



