156 HUNTING AND PISHING KACES 



The organisms which cause disease do not belong all to one 

 class. Some, such as the bacteria, are plants, others are animals. 

 Further, those species which cause disease are closely related to 

 other quite innocuous species. Non- virulent diphtheroid bacteria 

 are, for instance, found in the throat. What has happened has 

 been that certain species, belonging to different groups in the 

 animal and vegetable kingdoms, have taken to a parasitic mode 

 of life. It is not very difficult to imagine how this could occur. It 

 is to be presumed that organisms now parasitic were once free- 

 living and saprophytic. There is a bacillus which lives on grass ; 

 it is closely related to the tubercle bacillus but is harmless. Such 

 an organism might be frequently swallowed ; if liable to destruc- 

 tion, resistance might be evolved. 1 Its presence might be inno- 

 cuous, or it might not be so and a new disease might be the 

 consequence. The invasion of a host by a, parasite appears usually 

 to be followed by a struggle on the part of the host to get rid of 

 the parasite, and on the part of the parasite to maintain itself 

 within the host in the course of which struggle the parasite may 

 develop activities noxious to the host. To this struggle there are 

 different possible solutions. But, when we find the struggle in 

 progress, it is a fair assumption that the association of host and 

 parasite is recent. The very fact of disease, therefore, suggests that 

 it is of relatively recent origin. 



There are two further facts which are relevant in this connexion. 

 Diseases are very rare among species in a state of nature. Such 

 diseases as we know of among animals and plants occur for the 

 most part among domesticated species. Again, there are certain 

 conditions which favour the evolution of disease ; these conditions 

 are found among domesticated species and also among civilized 

 man. There are many ways in which disease can be transmitted 

 by insects, by liquid particles in the air, by drinking water, by 

 water used for bathing, by water introduced within the tissues, 

 and so on. Transmission by all these means, with the partial 

 exception of transmission by insects, is obviously very greatly 

 favoured by the aggregation of the members of the host species. 

 In fact only when such aggregation occurs can we understand 

 how disease on a large scale could be evolved. The aggregation of 

 men, such as occurred within the third period, into villages and 

 towns, gave rise to conditions under which bacteria could flourish 



1 Cohen, Physiological Therapeutics, vol. v, p. 158. 



