88 VARIABILITY 



' trained ' to attack living beings ; that is, they may be trained to 

 produce toxins so protective to themselves, and, therefore, so 

 injurious to the cells of the host, that they are able to live and 

 multiply in the dangerous environment till the host dies or acquires 

 immunity. This training, this conversion of a saprophytic into a 

 parasitic type, always involves placing the microbes in contact 

 with living cells. For example, streptococci, which are normally 

 saprophytic, may be rendered very virulent (i.e. capable of main- 

 taining existence in living tissues) by passage through a series of 

 animal hosts, but cannot otherwise be rendered virulent. On the 

 other hand, purely parasitic organisms may be trained to sapro- 

 phytic habits of life. If placed in a suitable non-living medium (e.g. 

 broth) where they are no longer persecuted by the cells, they slowly 

 lose their toxins, and, while becoming more capable of existing in 

 that particular medium, become less capable of maintaining them- 

 selves in the living body. In either case the change, since it 

 increases in successive generations till fitness to the new environ- 

 ment is achieved, is both germinal and adaptive. So close is the 

 adjustment to the environment that, usually, each microbic disease 

 is limited to a single species of animal. Thus most human diseases 

 are peculiar to man. When more than one species is attacked by 

 the same disease, as is often brought about experimentally, the 

 microbes tend to separate into varieties or races, each of which is 

 most virulent (i.e. best adapted to, best protected in) the species it 

 has made its host. Each parasitic species has its peculiar toxin, 

 a means of defence which is always the same for the same species, 

 but which differs from that of every other species, as may be judged 

 from the symptoms of the maladies they produce. Thus, strep- 

 tococci always produce the same symptoms, the same toxins, when 

 they become parasitic. The existence of the toxin, a very complex 

 chemical substance, implies a remarkable producing apparatus. 

 The evolution from saprophyte to parasite is mainly the evolution 

 of this apparatus. Unless we assume that the microbes of disease, 

 highly specialized organisms, were miraculously created to afflict 

 the higher species, we must assume that they were all evolved from 

 saprophytic types. 



141. Now we reach the problem as to the mode by which 

 saprophytic species become parasitic. It is probable that 

 all unicellular species, even the saprophytes, have some sort of 

 toxins by means of which they dissolve and digest their food or 

 protect themselves from other unicellular organisms, and which, 

 like the saliva of snakes, is capable of undergoing evolution till it 



