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Burdon Sanderson that the discharges of typhoid fever and 
cholera patients gain in virulence for a few days after their 
emission from the body, and Prof Pettenkofer’s Aground’ 
theory of the latter disease seems to demand the possession by 
disease germs of some such property as this ; bnt no one has 
hitherto ventured to suggest that a seclusion of several years 
enhances the powers possessed by the materies morbi of 
these disorders, 
5. Equally problematical is the suggestion that each 
epidemic is like a gigantic but disjointed organism, with a 
life history of the whole as well as of its individual parts. 
This notion would be on a par with Mr, Butler’s 
humorous idea that the “ ovum when impregnate should be 
considered not as descended from its ancestors, but as being 
a continuation of the personality of every ovum in the 
chain of its ancestry.” ^ 00^0^ 0^20 Qf being 
“ actually the primordial cell which never died nor dies, but 
which has differentiated itself into the life of the world, all 
living beings whatever being one with it, and members one 
of another.” 
6. It would be more nearly in accordance with our know- 
ledge of the ways of the lowest orders of beings if we were 
to suppose that after a certain lapse of time in which the 
disease germ was propagated by fission and by budding, 
it at length reached a period at which an act of conjuga- 
tion was necessary for its continued existence, and that 
after this process the disease regained its former powers of 
offence and virulence. 
7. I am inclined to think, however, that a much simpler 
explanation of the facts can be given than those hitherto 
brought forward, and this is, that a certain density of the 
population at susceptible ages is necessary before a disease 
can spread with the vigour of an epidemic. 
Probably all the facts would be accounted for if we 
suppose that these disorders can only become epidemic 
