28 TAXONOMY 



by almost all operations injurious to life, if only the proper degree of 

 strength be chosen. Anthrax bacilli are attenuated by a few hours' 

 exposure to direct sunlight or by the addition to the culture media of o-i- 

 0-2 per cent, phenol. Trichloride of iodine is effectual in the case of the 

 diphtheria and tetanus bacilli. By careful exposure to high temperatures, 

 Pasteur was able to produce a less virulent breed of anthrax bacilli which 

 retained their acquired characters for a long time (17). The change effected 

 was the same whether the cultures were maintained at 42-5 C. for twenty- 

 eight days, at 43 for six days, at 47 for four hours, or at 52 for fifteen 

 minutes. The temperature optimum for Bac. antJiracis is between 30 and 

 37, the maximum about 42 or 43, and the lethal limit lies between 50 

 and 60, so that it is evident that attenuation is effected the more quickly 

 the nearer the temperature approaches the lethal limit. 



That the decrease of virulence is only one phase of a general weakening 

 of all the functions is shown by the fact that the bacteria in attenuated cul- 

 tures have lost the power of forming spores : have become asporogenousQfy. 

 At first sight this seems to be a result of the greatest importance. Spore 

 formation is one of the principal morphological features of the cell. If we 

 were really able to suppress it, and to suppress it so successfully that it never 

 made its reappearance even in the most vigorous cultures, we should indeed 

 have done a great thing, for we should have obtained nothing less than 

 the long desired proof of the hereditary transmission of acquired charac- 

 ters. But appearances, alas, are deceptive. It is no more possible to 

 obtain in this way a breed of sporeless anthrax bacilli than it is to rear 

 a tailless race of mice by incessantly cutting off their tails. All we do 

 is to hinder the production of fully ripe, resistant spores : rudimentary 

 spores are formed. We attain nothing more than a general enfeeblement 

 of the cell affecting all its functions equally. This is proved by the fact 

 that such ' attenuated ' and ' sporeless ' cultures get weaker and weaker 

 and finally die. By the inoculation of the weakened bacteria into suitable 

 animals and several passages through the body, in short, by a bracing treat- 

 ment, virulence is restored and resistant, fully ripe spores appear again. 

 The micro-organisms behave, in fact, just as sickly plants do when removed 

 to healthy surroundings. The importance of the experimental modification 

 of virulence for the attainment of artificial immunity will be considered in 

 a later chapter. 



All these considerations show us that external agencies (in the brief period 

 which our experiments allow) are unable to bring about a lasting, hereditary 

 change in the structure of bacteria. The morphological characters remain 

 constant. The variety always throws back to the typical form, the concep- 

 tion of species and genera having the same value among bacteria as among 

 the higher organisms. Billroth's views, according to which all the bacteria 

 occurring in an infected wound were different developmental stages of the 



