130 BIOLOGICAL SIGNIFICANCE OF ANTITOXIN 



difficulties in the supposition, and it certainly favours the 

 explanation of the phenomenon. 



It will be remarked that the tendency of modern thought has 

 been rather to minimize the importance of antitoxin in the process 

 of natural recovery and in the subsequent immunity, and to regard 

 its appearance rather as an epiphenomenon, though doubtless one 

 that may under certain circumstances be of advantage to the 

 patient. Let us consider briefly the probable significance of anti- 

 toxin-formation in the animal economy. Is it a purposive pheno- 

 menon, developed and selected by natural selection with a view to 

 defend the species against natural dangers ? This was the natural 

 assumption when antitoxin was thought to play a role of the 

 utmost importance in natural recovery and acquired immunity. 

 There are great difficulties in the way of accepting such a hypo- 

 thesis. In the first place, immunity to bacterial infections is more 

 prevalent in the lower rather than in the higher animal types. 

 Doubtless very great susceptibility to a widespread infective 

 agent would operate unfavourably to the prospect of survival of 

 any animal species, but actual experience seems to show that 

 immunity to ordinary infections is a far less potent factor in 

 evolution than those studied by Darwin and his school. It would 

 appear in the case of tetanus at least that susceptibility is acquired 

 as the animal rises in the zoological scale as a secondary, though 

 perhaps necessary, result of the possession of a nervous system of 

 a certain degree of complexity. Secondly, the immunity due to a 

 successful struggle against a disease is an acquired factor, and as 

 such not transmitted, according, at least, to the majority of modern 

 authorities. It might be objected that the capability of producing 

 antitoxin might be a spontaneous feature, and one capable of being 

 transmitted by heredity. This is probably true, but in this case it 

 could only become a powerful agent in evolution if the disease 

 were prevalent and an attack usual in the life-history of many of 

 the individuals. This might occur in the case of tetanus, though 

 here, as we have seen, the tendency in evolution is for the production 

 of susceptibility rather than immunity ; but it is quite impossible 

 to see how the power of forming an antitoxin to eel serum after 

 the injection of that substance (to take one example of many 

 which might be quoted) can be of advantage to any animal unless 

 its habitat happens to be a pathological laboratory. Lastly, if the 

 acquirement of immunity be of great importance in natural 

 selection, we should expect those species to survive which de- 



