EVOLUTION 



"3 



But disease is much older than man, for Moodie has noted the presence of 

 arthritis and osteomyelitis in cave bears, of pyorrhoea in early tertiary three- 

 toed horses, and caries in Permian fish. 



There can be no doubt as to the great antiquity of bacteria, and it seems 

 possible that they performed great geological works in the early history of the 

 world, and may be largely responsible for the formation of some of the oldest 

 sedimentary rocks, in much the same way as Drew's Bacillus calcis does its 

 work to-day in the lagoons of Florida and on the Great Bahama Reef. 



When higher plants and animals evolved, the struggle for exist- 

 ence must have compelled minute animal and vegetal organisms to 

 seek protection for themselves therein, and at first this protection 

 would be used temporarily to tide over some difficulty. 



This being admitted, the parasite would desire an easy method 

 of entrance into and escape from the body of the temporary host, 

 and hence the common infection of the alimentary canal with 

 organisms. 



So long as the chemical substances produced by the metabolism 

 of the parasite were innocuous or helpful to the host, there would 

 be no reaction on its part against the intruder, and the two would 

 live together in peace, as many bacteria do at the present day in the 

 human alimentary canal, and a condition of commensalism may be 

 arrived at. 



Other intruders, on the other hand, might give rise to irritating 

 chemical substances which would provoke a reaction on the part of 

 the host. And thus begins the long struggle between the invading 

 organism and cells of the host which has produced all those compli- 

 cated mechanisms which are gathered together under the term 

 ' immunity.'^ .The reader will notice that the whole of this struggle 

 is adaptation to environment — viz., to that portion of the host's 

 environment in which is centred the invading organism. 



The parasite would naturally attempt to escape from the defensive 

 chemical substances poured on to it by the host, and in so doing 

 pierced more deeply into the tissues of that host ; but now its escape 

 would not be so easy as when living in some cavity freely communi- 

 cating with the exterior, and hence it would be to its advantage to 

 either kill that host, or, if it failed to do that, to enter some cavity 

 communicating with the exterior and suitable for the parasite's 

 existence — e.g., the enteric bacilli in the gall-bladder and the pelvis 

 of the kidney. 



All these changes of environment would cause variation in the 

 protists (protozoan and bacterial), and, if the same environmental 

 conditions acted long enough, then these changes would become 

 inherited {vide Adami); hence the origin of the numerous parasitic 

 protists, animal and vegetal, and hence also the preservation of 

 characters, including those complicated phenomena associated with 

 the reaction of the host which we call the signs and symptoms of 

 disease and the natural recovery therefrom It appears to us that 

 ■ ust as disease arose de novo in the long ago, so it has probably been 

 arising century after century, and we see no reason why new diseases 

 should not appear in the twentieth century of the present era in 



