224 THE PRESENT EVOLUTION OF MAN — PHYSICAL 



such can be made, to the influence of an altered en- 

 vironment in an artificial medium ; but great difficulties 

 will lie in the way of success by the latter method, 

 since syphilis, so far as we know, is a purely parasitic 

 disease, never a saprophytic one; that is, the micro- 

 organisms which produce it are capable of existence in 

 a living host, but not, so far as we know, in non-living 

 nutrient media. The difiiculty in the way of success 

 by the former method lies in the fact that we know 

 of no animal capable of taking the disease and 

 attenuating it. 



In diseases which have a shorter incubation period 

 than rabies — i. e. in which the toxins are more rapidly 

 produced — but in which the micro-organisms do not 

 pervade the body, but are localized in a definite area 

 of limited extent — e. g. in diphtheria and tetanus — it is 

 only reasonable to assume that the toxins elaborated 

 by them, however much they may paralyze or destroy 

 the phagocytes in the immediate neighbourhood of the 

 local lesion, where they are in a state of concentration, 

 however much in the concentrated state they may 

 ward off the attack of the phagocytes on the microbes, 

 do not at first destroy or paralyze the phagocytes in 

 the blood and tissues at a distance where they are in 

 a state of great dilution ; and therefore, as regards these 

 diseases also, we may hopefully attempt to cause re- 

 covery and produce acquired immunity by introducing, 

 before the virulent toxins reach such a degree of con- 

 centration as to destroy or paralyze the phagocytes, less 

 virulent toxins obtained from animals — e.g. horses — 

 which "attenuate" their microbes, as the microbes of 

 rabies are attenuated in the monkey, and as the microbes 

 of small-pox are attenuated in the calf, or else from 

 artificial cultures, in which the microbes are attenuated, 

 as the microbes of rabies are attenuated, in a slowly 

 drying cord, and as the microbes of anthrax are 



