THE BACTERIOLOGICAL EVIDENCE 89 



becomes poisonous to other forms of life. 1 Saprophytes cannot, 

 under normal circumstances, contend against the living cells of 

 multicellular types ; but that the toxins are strong in at least 

 some of their species is proved by the fact that the organisms, for 

 example, which cause putrefaction in dead bodies may, under 

 favourable conditions, infect and by poisoning destroy men and 

 other animals. 2 Unicellular organisms multiply very quickly, as 

 may be judged by the rapidity with which an epidemic of infectious 

 disease may sweep a large and populous country. Many genera- 

 tions elapse, therefore, even under the most favourable experimental 

 conditions, before a saprophytic species becomes truly parasitic. 

 Now, in this passage from an environment to which they are 

 adapted to another which at first is very unfavourable, do the 

 microbes make useful ' acquirements ' which are ' transmitted ' and 

 accumulated in descendants ? Or do they alter through the Natural 

 Selection of favourable spontaneous variations ? The former is the 

 accepted hypothesis. Indeed, bacteriologists do not seem aware 

 that there is an alternative. Reasoning from the analogy of 

 the human being, they suppose, in effect, that microbes are able 

 to make advantageous acquirements under the stimulus of use, and, 

 further, that, being unicellular, they are capable of transmitting 

 these acquirements to offspring, so that the latter start develop- 

 ment where the parents finished. 



142. But it is necessary to exercise a little scientific imagination. 

 The power of making use-acquirements is apparently a late product 

 of evolution, limited to the higher species, in which alone it is per- 

 ceptible. At any rate we have no right to assume thus easily that 

 lowly microbes possess it. Moreover, since the microbes of disease 

 begin by multiplying and flourishing greatly in an infected person, 

 manifestly they have at first much the best of the struggle, and 

 would keep it did they grow stronger under the stimulus of use. 

 Indeed, it is evident that, if microbes not only made use-acquire- 

 ments but transmitted them, then, since their multiplication is so 

 rapid, their virulence would soon become so exalted that the 



1 The evolution of a toxin implies the power, not only of producing it, but 

 of resisting it. Thus the cells of the poison gland of the snake are immune to 

 the venom, the cells of the alimentary canal are resistant to the digestive ferments, 

 the body-cells to the poisons which destroy the microbes. So, also, the microbes 

 are resistant to their own toxins. 



2 It is said that the carnivora are particularly immune to septicaemia. If 

 this is true, it is an interesting example of special evolution. Owing to the 

 nature of their food and the wounds they are apt to receive in pursuit of it, 

 carnivora are particularly exposed to infection. 



