TiXlV. 



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 will lead 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 to a 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, e.^., 

 the red corpuscles which are not so very much like living animals, and the 

 colourless corpuscles, which are exactly like those little creatures called 

 amoebae. 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 



