SCOTTISH METROPOLITAN VETERINARY MEDICAL SOCIETY. 503 
With many of the diseases—according to some authorities, with all 
—certain organisms have been discovered in the blood and other fluids, 
which are looked upon as the special agents in producing the charac¬ 
teristic effects that mark each disease. These organisms are the so-called 
“germs;” and the theory which now generally finds acceptance among 
the more advanced pathologists, with the regard to the production of 
these maladies, is named the “ germ theory.” This theory is certainly 
the most satisfactory which has ever been proposed to account for the 
production, course, and extension of these diseases; and it is supported 
by all the facts that present themselves in our study of these affections. 
The particles or germs of a contagium may well and aptly be compared 
to the germs in the seeds of plants. No plant can be produced without 
seed, and the germ in this seed must be endowed with vitality—it must 
be living. The seed requires a congenial or suitable soil, and other 
accessory conditions, for the growth of its germ ; but when buried in 
the soil, and placed in the most favorable condition for development, 
it does not at once become a plant. There is a definite latent or germi- 
native period in the evolution of every plant, during which nothing is 
seen, but, nevertheless, germination is progressing; then comes the 
eruptive period, when, certain changes having been accomplished, the 
young plant becomes visible; then arrives the stage of blossoming and 
seed production—the seeds ripen, and every one of them can again, 
under suitable conditions, reproduce the parent plant and multitudes 
of germinal seeds. Every seed becomes converted into a plant exactly 
like that which produced it: the seed of a carrot will not produce a 
turnip, neither will an oat develop a thistle. 
“Like genders like, potatoes Tatoes breed, 
Uncostly cabbage springs from cabbage seed.” 
The seed of each plant has its own special phases of evolution, which 
are as peculiar to it as its physical or other attributes, and man can 
effect but little in modifying these. 
Zymotic or contagious diseases, such as those of which I am now 
speaking, bear the strongest analogy to plants in this respect. Each, I 
assert, has its own special germ, which can only develop its own parti¬ 
cular disorder—never any other. This germ can no more be spoil- 
taneously developed than can the seed or the plant. It has a life, an 
individuality of its own, which nothing can transmute or alter. As the 
carrot seed will not grow into a turnip, nor the oat produce a thistle, so 
neither will the germ of cattle plague develop foot-and-mouth disease, 
or that of rabies give rise to variola. The germs of each zymotic malady 
are as distinct and independent as different kinds of plants or different 
kinds of animals; and these germs can be no more transmuted into each 
other than can the plants or animals. The organism of a susceptible 
animal receives the germs of a particular disease as the ground receives 
the seed—there is a characteristic period of latency or germination, 
during which the most careful observer can detect no change; then 
comes the eruptive period, with the manifestation and definite course 
of the symptoms, which characterise the malady as distinctly as the 
leaves, the flowers, &c., characterise the plant when the multiplication of 
new germs has reached its maximum stage, and these can infect any 
number of other susceptible creatures into which they may be sown. 
Another feature in these zymotic diseases gives them an additional 
resemblance to the growth of seeds. One attack protects against 
another, for perhaps years, if not for life—the germs will no longer grow 
in that soil, and the organism is endowed with immunity. We know 
that the rotation of crops is an absolute essential in agriculture; the 
