THEORIES OF EVOLUTION 21 



are extremely variable as is shown by numerous changes of 

 form under conditions which preclude the possibilities of 

 these alterations in shape and size being mere modifica- 

 tions. Thus the descendants of a single diphtheria bacillus, 

 grown in broth, may be comparatively large or small; 

 straight, clubbed, or branched; or the descendants of a 

 single vibrio, grown under similar conditions, may be 

 spirillar or S-shaped. By judicious management purely 

 parasitic organisms may be rendered saprophytic, and 

 vice versd. As parasites they gradually become resistant 

 to the germicidal action of the tissues ; as saprophytes they 

 slowly grow resistant to the germicidal action of the outer 

 environment. If quickly transferred from one environment 

 to the other, they tend to perish. If slowly transferred to a 

 non-living medium, parasitic organisms tend to lose their viru- 

 lence ; whereas the virulence of saprophytic organisms may 

 be enormously exalted by passage through a series of animal 

 hosts, as in the case of streptococci. In some cases they 

 cannot be passed from one medium to another except by 

 way of a third. In all these examples the time occupied 

 by the change is ample for the operation of Natural 

 Selection. 1 Consequently the changes, which bacteriologists 

 have attributed entirely to the transmission of acquirements, 

 may be due altogether to Natural Selection. 



35. Indeed in many cases it is practically certain that 

 Natural Selection is the God in the machine. Thus toxins, 

 extremely complex chemical compounds, are defensive 

 weapons which protect the organisms producing them from 

 their enemies, the phagocytes of the blood and tissues. It 

 is possible to imagine that the evolution of the power of 

 producing toxins is due to the survival in comparative 

 numbers of those individuals who happen to produce the 

 more poisonous secretions ; but it is difficult to imagine its 

 evolution through the transmission of acquirements. How 

 could such remarkable acquirements be made ? Why are 

 the poisons specific, and not of a type common to all the 

 species? Why do they arise only in species exposed to 

 particular dangers, and not in the others ? Through the 

 effects of use ! But what kind of use ? A bacillus is not 

 an experimental laboratory which conducts improvements 



1 Organisms (e. g. bacillus typhosus) which normally alternate be- 

 tween a parasitic and saprophytic existence, must not, of course, be 

 confused with the cases given above. The alternation here is com- 

 parable to the alternation of generations. in higher forms, and probably, 

 like it, has been evolved by Natural Selection. 



