same, chiefly involving greater degrees of complexity in the latter, and that we must apply the same principles for the recognition of disease in each case. This view was expressed so long ago as 1846, and receives confirmation in the expressions of some of our best pathologists of the present day. Frank tells us that "disease is every deviation from the normal condition of the species"; while Sorauer says, "We must recognize as a disease every disturbance of the organism which detracts from the final end of its labor, the accomplishment of its purpose."
In considering the diseases of plants, it is important to bear in mind that we have to deal with subjects which on the one hand are cultivated, and on the other hand not. In forest-trees there has been no modification through cultivation, and disease would not be likely to become complicated from this cause. In cultivated trees, and plants, as in the peach, pear, strawberry, raspberry, etc., a high degree of cultivation has resulted in a corresponding modification upon which the pecuniary value directly depends. This strong divergence from the original type involves a debility in one or more directions, and is quite parallel with the changes known to occur in more highly civilized communities of men, by reason of which diseases are not only likely to be more prevalent but more complicated. This analogy, as well as general principles, would show us that the more highly cultivated the varieties of fruits or plants, the more susceptible are they to the influence of environment with the introduction of disease, and this is confirmed, not only by personal observation, but by the experience of practical fruit-growers.
Again, cultivated fruits always tend to revert to the original form when the conditions of their high state of development are withdrawn. Moreover, such organs often show that this excessive development has obliterated, wholly or in part, those important functions connected with the reproductive processes which they were originally designed to fulfill. These are some of the evidences that all such monstrosities as our modern apples, pears, strawberries—in short, all our cultivated fruits—are in reality abnormal growths which we may designate as hypertrophied structures, and are therefore evidences of disease. In such cases, therefore, the questions of treatment are likely to become somewhat complicated, since, while maintaining a certain form of disease, we must exclude, prevent, and cure all others.
Diseases may be general in the system, or they may be localized, and this is a consideration of obvious importance when we bear in mind that, according as they are one or the other, they may be more or less destructive in their effects or be controlled with greater or less
difficulty. When a disease involves the entire system, as in peach-yellows or pear-blight, it is often a matter of great difficulty to deter-
- Smee on "The Potato Plant."
- "Lancet," 1880, vol. ii, pp. 605, 645.
- "Krankheiten der Pflanzen," p. 2.
- "Handbuch," p. 56.