president's address. 213 



is that of inoculating with the bacillus slips of prepared gelatine, 

 which affords a suitable nidus for the growth of the organisms. 



The variety of form presented by Eacteria is very great, and it 

 was at one time believed by not a few observers that these varia- 

 tions indicated only phases of the life-history of a small number 

 of species, or even that the whole series represented embryonic 

 conditions of the higher Algae or Fungi. The reproductive pro- 

 cess in some of the Bacteria has now, however, been sufficiently 

 observed, and there can be no doubt that they are really mature 

 organisms, that there are large numbers of species, and that 

 though the same species is often capable of adapting itself to 

 varied conditions, their vital requirements are yet very diverse. 

 Thus some forms need oxygen for their nourishment, others live 

 and multiply entirely without it : some are destroyed by small 

 traces of acid in their nutrient fluid, others require the presence 

 of acid : the products of their vital activity are also, chemically, 

 very various, and are doubtless in many cases the chief cause 

 of their deleterious effects on the animal organism. The repro- 

 duction of the Bacteria, apart from growth by fission which is 

 constantly going on with extreme rapidity, consists in the for- 

 mation of a spore which becomes encased in a tough and dense 

 envelope, and is thus enabled to resist adverse conditions such 

 as drought and heat, which would speedily destroy the parent 

 form. It has been proved that spores will, at any rate in some 

 cases, resist temperatures of 100°, or even 130° C. (212°— 266° 

 Fahr.). But it appears that the formation of spores does not 

 occur so long as the environment is favourable to the growth of 

 the Bacteria ; when these conditions fail and there is a prospect 

 of death and extinction before the adult, then the case-hardened 

 spores begin to be produced ; and the fact of their great resisting 

 power as to heat and other agencies is a sufficient explanation 

 of the apparent success of many experiments in what has been 

 called "spontaneous generation." 



The classification of Bacteria presents very considerable diffi- 

 culty, many systems having been proposed, based upon the form 

 of the organism, its mode of reproduction and nutrition, or the 

 products of its vital action. It will be sufficient here to state 



