ADAPTIVE VARIATIONS. 387 



solutions of the poison of increasing strengths, and 

 after feeding for various lengths of time, the maximum 

 amount of poison the animals could withstand was de- 

 termined. This amount rapidly increased after the 

 first day, so that after three weeks' feeding it was found 

 to be no less than 200 to 800 times the original dose. 

 Some of these mice were then kept on normal food for 

 over six months, and at the end of that time could still 

 withstand considerably more than fifty times the origi- 

 nal amount of poison. 



Even more remarkable results have been obtained in 

 the preparation of diphtheria antitoxin. For this pur- 

 pose, Roux * uses the filtrate from diphtheria bacillus 

 cultures, it being at first mixed with an iodine solution 

 to reduce its virulency. One-quarter cc. of the iodised 

 toxin is injected on the first day, and this is increased 

 to 1 cc. on the 13th day. On the 17th day \ cc. of 

 the pure toxin is injected, and this is gradually in- 

 creased in amount till on the 41st day 10 cc. is in- 

 jected, and on the 80th day no less than 250 cc. 

 The virulency of the last dose must have been some 

 5000 to 10,000 times greater than that of the first dose, 

 and, supposing the effect produced on the horse was 

 more or less the same after each injection, its acclimati- 

 sation to the toxin must have increased in similar pro- 

 portion. As is well known, animals can be acclimatised 

 to toxins produced by other bacteria, such as those of 

 anthrax, tetanus, cholera, typhoid, plague, and likewise 

 also to snake venom; but it is unnecessary to refer to 

 these here. Upon acclimatisation in man there are 

 probably no exact observations, but the inexact and un- 

 * Vide Crookshank's " Text-Book of Bacteriology," 1896, p. 58. 



