360 THE CAUSATION OF DISEASE. 



because such families afford the fitting soil. Let him who is 

 inclined to scoff at the " soil " theory explain the preferences of 

 other parasites for particular individuals and for particular bodily 

 states. Let him tell us how it is that ordinary skin parasites 

 show undoubted partialities — that, of two individuals equally 

 exposed to the same bacterial influence, one is stricken with 

 fever while the other escapes unhurt; that the heads of 

 patients who have been tended with the utmost care will 

 sometimes of a sudden swarm with vermin ; and that, as death 

 creeps upon an individual, myriads of bacteria, hitherto quite 

 powerless to hurt him, swarm the tissues, beginning their work 

 of destruction ere life is yet extinct. These and kindred facts 

 can only be explained by supposing that certain states of the 

 body are particularly favourable to parasitic growth. Thus, 

 also, we can explain the tendency of cancer to run in families, 

 and to occur after middle life. 



The comparative infrequency of the malignant growths is 

 not, in my belief, due to the rarity of the parasites causing 

 them, but to the happy infrequency of soil favourable to the 

 growth of the parasite ; and this fact must be borne in mind 

 in attempts at inoculation, and although all such attempts have 

 failed, let it be remembered (a fact which is apt to be for- 

 gotten) that this failure is equally difficult of explanation, 

 whether the parasitic theory stand or fall, for certain it is that 

 the body inoculates itself. I say the tendency to malignant 

 disease is due to the power of growing the parasite, and not to 

 the affected tissues having any particular and special tendency 

 to undergo the malignant change, for I hold that the tissues of 

 all alike possess these potentialities. Exemption from malignant 

 disease depends, in fact, upon exemption from the evil cell-E, 

 and this, again, upon some peculiar body state unfavourable 

 to the bacterial cultivation, and not upon any inability of the 

 tissues to take on malignant change should they be subjected 

 to the necessary mal-E. The capacity to undergo malignant 

 dissolution belongs to all vertebrates : all possess the same 

 potentialities, and what wonder? All the members of this 

 vast kingdom are knit together under the one grand scheme 

 of evolution. At every onward step there is divergence — a 



