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of plants yet to be discovered, nor to discourage the search for these. 
It is rather to emphasize that there are other problems better worth 
while than the search for "new" diseases of minor economic im- 
portance. The simplicity of the bacteria in their relations to host 
and in the way they lend themselves to culture and infection stimu- 
late the hope that through persistent intensive study of bacterial 
diseases we shall gain the clearer insight into those intimate relations 
of parasite and host which are fundamental to the science of plant 
pathology. 
VI. The Relations of Parasite to Host and Environment 
Although parasitology in relation to plant pathology dates from 
but little later than in animal pathology; and the relations involved 
would seem simpler than with the animal parasite, yet the fact remains 
that we are far behind the animal pathologists in understanding these 
relations. Some of the reasons for this are evident. The preeminent 
value of human life among the animals has focused attention upon 
human pathology. Even where attention has been given to the 
pathology of the lower animals, the students have as a rule approached 
the subject from the viewpoint of human pathology, and have been 
eager to apply to this any suggestions from comparative work on the 
lower forms. The result has been intensity and concentration of 
research upon the diseases of this one organism, man. 
In plant pathology the natural tendency has been exactly the 
opposite. From the beginning the phytopathologist has included in 
his range of interests all the diseases of all plants known to him. The 
numbers of disease-inducing parasites is so enormous that it has con- 
sumed his professional energies simply to catalogue them. Concen- 
tration when attempted has been secured by narrowing one's interests 
within the parasitic group rather than within the host group. 
I believe that we need to have, far more than heretofore, special- 
ization by hosts in our phytopathological studies. Whether one is to 
probe deeper into problems of relations of environment to parasitism 
or into matters of predisposition and variations, either with host as to 
susceptibility or parasite as to its biological forms, attention should 
be focused long and intensively upon the one host. Experience has 
convinced me that one cannot understand the diseases of a cultivated 
plant like the potato, for example, except as he understands them in 
