— 8 — 
of trees affected by blight, and their presence there was 
considered as sufficient evidence that they caused the dis- 
ease. No crucial test was ever applied to prove that causal 
action. So in the absence of positive proof, all the claims 
of discovered cause made, up to this time were valueless. 
DISCOVERY OF THE TRUE CAUSE. 
The first light shed upon what has since been proved to 
be the true cause of pear blight was in 1878 when Professor 
Burrill of Illinois announced to the Illinois State Horticul- 
tural Society the discovery of bacteria apparently connected 
with the disease. The germ theory of disease had been 
under discussion for several years, and, previous to this 
time Pasteur had (in 1869-70) demonstrated that a microbe 
caused the terrible silk-worm disease, and later in 1876 that 
splenic fever and fowl cholera were also due to the action 
of specific microbes. Professor Burrill was the first to sug- 
gest that these low organisms might be connected with plant 
diseases. In his announcement in 1878 he made no positive 
assertion, but simply reported discoveries which were suffi- 
cient foundation for a very strong suspicion that these 
organisms did cause the disease. Continuing his investiga- 
tions of the subject, in 1880 he had advanced far enough to 
announce before the American Association for the Advance- 
ment of Science that he had discovered the cause of pear 
blight. That the cause was a specific organism, for which 
he proposed the name Micrococcus amylovorus. Professor 
Burrill rested his claim upon the results obtained in a series 
of experiments. He inoculated healthy pear and apple trees 
with diseased tissue, and, in a large number of cases, blight 
followed the inoculation. The process of inoculation was 
both by the transfer of small pieces of diseased bark, and 
by pricking with a needle dipped in macerated diseased 
tissue. His results would seem to warrant his assertion 
that blight was caused by the organism which the micro- 
scope showed was present in large numbers. But in the 
light of modern methods of experiment, his proof could not 
be considered as absolute. 
Investigators of the etiology of the cont igious diseases 
of animals, agree, that in order to prove posi lively that any 
suspected organism is the specific cause ot any particular 
disease, four steps are necessary. These steps which were 
first recognized, enumerated, and published by Professor 
Cohn, are as follows : 
I. To demonstrate the habitual presence of the organ- 
ism in cases of the disease in question. 
