Blood, upon leaving the vein, is separated into a liquid and a solid part by fibrinogen, one of the proteids dissolved in it, undergoing a change, by which it becomes insoluble fibrin; muscular plasma, the semi-solid constituent of muscular fiber, by solidifying after death, and changing into fibrin, affords the well-known phenomenon of rigor mortis. It is a process closely resembling these, by which one of the soluble proteids of gluten, by accession of air and water, is transformed into insoluble gluten fibrin.
The conformity existing between the nitrogenous compounds of vegetables and animals is not limited to the proteids mentioned, but extends to various products of their decomposition. Diastase, the fermentative agent originating from proteids in the seeds of many plants during germination, is also present in the saliva of animals, where it exerts upon amylaceous foods the same action of forming sugar from starch. Agents very similar to pepsin, the digesting ferment of the stomach, are to be found dissolved in the juices of various plants; upon contact with muscular fiber, or coagulated albumen, or cheese, they will dissolve these bodies, and transform them into peptones as well as pepsin does. Thus papayotin, a substance extracted from the juice of Carica papayn—a kind of fig-tree—is therapeutically applied for dissolving morbid membranes and tumors; not less are the well-known insect-devouring properties of the leaves of Nepenthes, Drosera, and Utricularia, due to the presence of such ferments in their viscid secretions. Leucin and tyrosin, amides occurring in the roots of plants, which are both products of decomposition and regenerators of albuminoids, are also found in animal organs. The inflammation of skin, caused by touching a nettle, has been ascertained to be due to a kind of decomposed, or changed, proteid; and the virulent properties of the secretions contained in the venom glands of serpents having become known to depend on proteids, the conclusion appears well founded that the virus of insects and other animals also owes its pernicious effect to metamorphosed albuminoids, too, rather than to formic acid, as had been hitherto believed.
Many facts have been produced to show that, as the theory of evolution supposes, there is a degree of consanguinity existing between plants and animals: sexual differentiation, for instance, is common to the higher forms of both of them. Sexes in both cases are sometimes united in the same individual, sometimes separated. The lowest species of both of them, consisting of single cells, propagate by simple division. Scarcely a characteristic has been discovered in these living cells of plasma, which might justify the making of a distinction between plants and animals. The voluntary movement ascribed to the latter class is not plainly discernible in many cases, while many low plants, such as diatomaceæ and certain bacteria are eminently and continually engaged in lively motion. No striking difference is to be observed in the sources of food and in the way in which it is
