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That life should manifest itself certain conditions are essential, but some of which, though generally present, are not absolutely indispensable. One condition, however, seems to be very necessary, and that is that the living body should be composed of a certain material. This material, which forms the essential and fundamental parts of everything living, whether vegetable or animal, is technically called protoplasm. Other substances than it are often found in living bodies, but it is in protoplasm only that vitality appears to be inherent.

But whether it is the same in plants as in animals is a matter of opinion. One thing, however, seems reasonably certain, and that is that it is the medium or vehicle through which vital force is made manifest. Used in its general sense, protoplasm is chemically related in its nature to albumen, and generally has the character of a jelly-like, semi-fluid, transparent material, which, in itself, exhibits no definiteness of structure. When heated to a certain temperature it coagulates, just as the white of an egg does when boiled. Living protoplasm has the power of movement, of increasing in size or of maintaining its existence by the assimilation of fresh and foreign materials, and of detaching portions of itself which may subsequently develop into fresh masses. Though protoplasm be present in the ova of animals and the seeds of plants, yet there is no external and visible manifestation of life. There is in them what is called a dormant vitality, which may remain for a long time unchanged, until altered external circumstances cause the organism to pass into a state of active life.