taken. If to these facts the results of chemical investigations be added, showing that many constituents of the greatest vital importance in both are identical, and that a close relationship of others (vegetable and animal fats) is beyond doubt, we may justly claim that chemistry has contributed and will further contribute considerable support to the doctrine of evolution, if identity of matter may be regarded as evidence of consanguinity, or as a proof of common descent.
Considered from another point of view the mutual relations existing between both classes of beings are of a decidedly hostile character. The fierce "struggle for life," which causes animals to kill and to devour their fellow-creatures, is in a not less merciless degree extended to plants; and victory is not, as we might fancy, always on the side of the more perfect creature. Even man, the most accomplished being, and at the same time the most relentless despot on the earth, though without much personal trouble he may fell the strongest trees and eradicate whole forests, is yet liable to succumb to the attacks of a few micro-organisms invisible to the human eye. A theory, at first pointed out by Pasteur, accepted by Virchow, and of late experimentally confirmed by Metschnikoff, teaches, that certain low plants, fungi, called microbia, or bacteria, said to be the primary causes of infectious diseases, when entering the circulation of blood or one of the important organs of the body, become at once engaged in a struggle with the living cells of the organism, both adversaries endeavoring to kill and to devour each other; the result of this fight, if the microbia are victorious, is said to be the death of the animal.
In a lively and perspicuous representation Metschnikoff has described one of these destructive combats. Daphnia, a sweet water crustacean, served to him as the first object on which he could observe the attack by monospora, a fungus of the lowest order. As soon as the latter began to invade the body of Daphnia it became surrounded and entangled by numerous cells (leucocytes) engaged in lively motion, which gradually from all sides attached themselves to the fungus and destroyed it by some kind of intercellular digestion, or absorption. It is evident, that a single cell could not afford to give out so much dissolving matter as was required for this purpose; it would even probably have succumbed to the enterprising enemy; but by the assistance of its confederates it succeeded in overwhelming the intruder. In eighty cases out of a hundred, according to Metschnikoff, the cells would be victorious, but in twenty cases the fungus would gain the battle, with the consequent death of the Daphnia.
Concerning this view and interpretation of the origin and progress of infectious diseases, we are inclined to believe that, since bacteria have not been found in all of them, and since, where they are present, also certain products of chemical decomposition of proteids, ptomaines, occur with them—the dangerous phenomena and the lethal end of such diseases should rather be ascribed to the well-known virulent proper-