THE CAUSATION OF DISEASE. 237 



less accurately, if one may so put it, into his E ; and at what 

 prodigious sacrifice of life only those can realize who have 

 examined thoroughly the doctrine of evolution in its many 

 ramifications. And I doubt not but that one of man's most 

 malignant foes has been the one-celled organism, and that his 

 struggle against it has been severe in the extreme. One 

 would not, a priori, suppose that any unicellular organ- 

 ism was necessarily fatal to a multicellular being, for is not 

 the basis of life one and the same for all — viz., protoplasm ? 

 No fact of biology is more remarkable than that one form of 

 protoplasm should be capable of exerting a poisonous effect on 

 another. This effect is brought about by the emission of 

 certain poisonous matters. In pysemia, for instance, the evil 

 results from chemical matter given out by certain micro- 

 organisms. These chemical products are powerless to harm the 

 micro-organisms emitting them, but are, nevertheless, capable 

 of destroying many multicellular organisms — man among others. 

 Now, the fact that a certain chemical substance is harmless to 

 one form of protoplasm, but capable of producing disease in 

 another, is suggestive, at least, that this latter form might also 

 become capable, by adaptation, of withstanding its evil effect ; 

 and this suggestion is rendered all the more probable by the 

 fact that different species of animals differ most extraordinarily 

 in their manner of responding to different micro-organisms, for 

 what will produce rapid death in one will be quite harmless as 

 regards another : nay, more than this, a like difference may, as 

 we have already seen, be observed among the various members 

 of the same species. 



Now, all these facts tend to show that adaptation to almost 

 every variety of pathogenic micro-organism would be possible, 

 provided the latter were fixed in properly. The variability 

 in unicellular organisms has tended to prevent this, and, but 

 for this variability, it seems to me highly probable that 

 perfect adaptation to all forms of pathogenic micro-organisms 

 would long ago have occurred. Even as it is, this adapta- 

 tion has probably taken place to a large extent, and who 

 knows but that we may be living in the midst of an infinite 

 variety of micro-organisms which at one time were capable of 

 striking our ancestors (near or remote) with the most fearful 



