1901] MICROSCOPICAL JOURNAL. 135 
comparatively recent times, and when the dog had ar- 
rived at substantially its present form and development, 
or it may have been in some previous geologic age, when 
the conditions of environment upon all parts of the earth 
were far different from what they are at the present day. 
It is not to be supposed that the strange animals whose 
fossil remains prove their existence many thousand years 
ago were free from contagious diseases any more than are 
the animals which live to-day ; but whether the diseases 
of the prehistoric animal species were propagated from 
animal to animal until our time, or whether they disap- 
peared and were replaced by more recent plagues, it is 
now impossible to say. 
A study of the communicable diseases indicates that 
most if not all of them are caused by parasitic organisms. 
Indeed, the animal body has become the host of a multi- 
tude of parasites, most astonishing because of the num- 
ber of species and the great variety of forms. All of these 
parasites probably at one time in the existence of their 
species, or of the ancestors of their species, lived else- 
where in nature. Under certain conditions they were at- 
tracted to certain kinds of animals; they found they could 
live upon or within them; they adapted themselves to 
these new conditions ; their form and their physiological 
requirements were gradually changed, until finally in the 
course of time they could not exist elsewhere. They were 
then strictly parasitic. 
So far has this development and adaptation to the con- 
ditions of environment gone that we find different species 
and varieties of lice, of mites, and of worms living upon 
each different species of animals, and in most cases these 
parasites perish if transferred from one species of ani- 
mals to another species. If, therefore, these parasites 
can not exist when transferred to a different species of 
animals from that upon which they have developed and 
to which they have become adapted, there is all the more 
