226 BACTERIOLOGY 



completely changed its metabolism under artificial cultivation. The 

 more rational hypothesis seemed to be the one which assumed the 

 existence of some protective substance only slightly acted upon 

 by the serum, not at all. in glycerin bouillon, and therefore a hin- 

 drance to multiplication. After repeated transfers, this protective 

 substance was slowly lost, either through a selection of bacilli, or 

 absence of stimulation on the part of the host, or both causes com- 

 bined, and the growth became as luxuriant as with the more sapro- 

 phytic human varieties. It is obvious that each group or species 

 of bacteria would have its own special protective device, depending 

 upon original capacities of the species, which would be gradually 

 developed in power and efficiency with the perfection of parasitic 

 relations. 



The formation of protective or defensive coverings, the strength- 

 ening or modification of the cell-wall, or the secretion of defensive 

 fluids, would account for certain phenomena which are familiar to 

 bacteriologists much better than the current theory which bases 

 parasitism exclusively upon toxin production, active or passive. 



In cultures we should expect a loss of power to form protective 

 substances because the anti-bodies are absent. Hence the uni- 

 versal tendency toward the reduction and final loss of virulence, 

 with apparently the metabolic and vegetative activities unchanged, 

 and the frequently observed regaining of virulence by passages 

 through series of animals. 



In the evolution of parasitic bacteria I assume, then, that though 

 the function of toxin production may have been the entering wedge 

 toward a parasitic existence, there is a progressive loss of this func- 

 tion as of little use to the parasite after it has once acquired a foot- 

 hold, for the action of toxins at a distance from the focus of mul- 

 tiplication does not aid the parasite, while it may destroy the host. 

 In other words, with the invasion of the tissues of the latter it 

 became necessary for the invader to concentrate its powers in its 

 immediate vicinity, and for this purpose those poisons set free by 

 the disintegration of the parasite may be of use in protecting the 

 focus where the younger forms are, by necrosis of tissue, plugging 

 of vessels, etc., and thereby keeping away the bactericidal forces 

 until the bacteria have accumulated sufficient protective power 

 to subsist in a latent condition, and are read}^ to be discharged 

 outward. With the loss of active toxin production according to 

 this hypothesis, and the loss of other, now useless, metabolic activ- 

 ities, there goes hand in hand a strengthening of the defensive 

 functions. This strengthening I interpret as the gradual develop- 

 ment of certain substances which the non-immune host is unable to 

 act upon, or at most only in a slight degree. This substance, which, 

 as it were, shoves itself between the parasite and the common 



