THE CAUSATION OF DISEASE. 283 



strong and vigorous the body : but over and above such neces- 

 sarily-fatal-E's, there are numerous forms of E which, while 

 they induce disease in many, have not power to hurt all, or, if 

 capable of hurting, are incapable of destroying all. To such 

 E's adaptation is obviously possible by a process of natural 

 selection, and in this way numbers are carried off by rheu- 

 matism, heart disease, tubercle, and similar disorders, induced 

 by conditions which have no power to harm their more for- 

 tunate brethren. Such unlucky variations are, no doubt, often 

 enough the result of ancestral mal-E, but not necessarily, for 

 an individual may so vary as to become unduly susceptible to 

 some contagia — for instance, the tubercle bacillus, or other 

 similar agents incapable of working harm on the average 

 healthy individual — even though he and his more immediate 

 ancestors have been sound in body and healthy in mode of life. 

 The like is true of many diseases. Wherefore unfit variations 

 must necessarily occur from time to time under the most 

 perfect hygienic system, and a fortiori, under that quantity of 

 mal-E which is the common lot of all — the necessarily-mal-E. 

 Whatever, therefore, the far-off future may bring with it in 

 the way of hygienic advance, disease must ever remain a neces- 

 sary evil ; and, although much improvement will unquestion- 

 ably take place, let us bear in mind that the instability of E 

 will certainly tend to increase with the steady progress of 

 evolution, and that this instability is a great check to perfect 

 adaptation, whether affected through natural selection or 

 otherwise. 



