308 THE EVOLUTION OF LIFE 
Bacteria of to-day are heirs of all the ages, having 
descended from an untold number of generations, 
rather than being often, as I believe, things of 
yesterday—that is, units continually born anew, 
with recurring similar forms and properties, just 
as it happens with different kinds of crystals: 
the forms and properties in each case being the 
necessary and invariable results of the molecular 
composition of the units in question. 
But, even from the point of view of such doctrines 
as are now not uncommonly held by bacteriologists 
(many of whom are disposed to take such a view 
concerning the origin of Plague as I have quoted 
from Lehmann and Neumann), we may well ask 
why the tubercle Bacillus may not also frequently 
arise from other common Bacilli, when these gain 
access to lungs, joints, or glands in persons whose 
general condition predisposes them to scrofulous or 
tubercular affections. The change of doctrine con- 
cerning phthisis, to which I have previously referred, 
is altogether astonishing, when one thinks of the 
extreme paucity of direct evidence that exists in 
support of the prevalent view of the exclusive spread 
of this disease by contagion. 
It is now the fashion to suppose that the frag- 
mentation of dried sputa from phthisical subjects 
gives rise to the dissemination of contagious par- 
ticles, which, taken into the lungs of susceptible 
persons, set up the disease anew in them. It is well 
to look, however, more closely into such a view, and 
to ask what direct evidence there is in favour of this 
as a common mode of dissemination of the disease. 

