XXlv. 
characters of a particular kind of ‘virus,’ which multiplies in a most marvellous 
and rapid manner. Even so long ago as at the time of the ‘Great Plague’ 
of London, the belief was expressed that the pestilence was probably due to 
some living organisms, which entered the blood, quickly ‘multiplied in it, and 
passed from one individual to another, through the medium of the air; or 
still more certainly, if there should happen to be actual contact between the 
tissues of individuals already affected, and those of other people. In those 
days, however, men had not the means necessary for the discovery of the 
minute living organisms, which give rise to the diseases alluded to. At 
length, however, it has been demonstrated beyond doubt, that many diseases, 
such as, for instance, glanders, hydrophobia, anthrax, or splenic fever, 
tuberculosis, popularly known as consumption, that dread malady of man 
and beast (which is due to the Bacillus Tuberculosis), and others, are severally 
‘connected with, and therefore, in all probability, dependent on the presence of 
different kinds of vegetable fungi, of microscopic size, in the blood and 
tissues. Moreover, in the case of those fevers in which special germs have 
not as yet been satisfactorily demonstrated, there is but little reason to doubt 
that renewed and more searching investigation willlead to a similar conclusion 
as to their causation. There is, for instance, reason to suppose that dysentery 
will eventually be proved to be due to some living vegetable germs ; and,. 
although it is doubtful if cholera is due toa similar cause, it is most probable 
that the discovery of the virus of this disease is but a question of time. 
If we contemplate the fact that such horrible plagues among the higher 
animals, are caused by the inroads of myriads of certain specific germs, we 
shall find very much food for earnest reflection. In the first place we must 
remember, that all the higher organisms, both animal and vegetable, are in 
reality, composed of innumerable cells, which may to a large extent be 
considered as separate living units. More than this, we find the normal 
blood of a healthy animal containing millions of little cell-like creatures, ORG y 
the red corpuscles which are not so very much like living animals, and the 
colourless corpuscles, which are exactly like those little creatures called 
amoebze. Who can say what is the real significance of the presence of these 
small organisms in the blood,—for organisms they can be without doubt 
truly called? How do we know that they, originally living in the outside 
world, have not gradually succeeded in taking up their abode in the blood 
of the higher animals? This is one way of looking at the question, and it 
must be confessed that it is an admissible explanation of their presence. 
After many generations, according to this view, they have come to be 
essential constituents of the higher animals, and to subserve necessary 
functions. Of course such speculations are beset with difficulties. With 
regard, however, to the much more simple germs which give rise to disease, 
does it not almost look as if certain very low forms of life, happening by 
some combination of circumstances to be favourably implanted into the 
bodies of the higher animals, run a certain course of adaptation to their new 
conditions ? At first, being exposed to a new environment, they multiply 
rapidly to the very great detriment of their hosts. The latter at length react 
