When we behold the plants and animals that ordinarily surround us, the distinction between the animal and vegetable kingdom is somehow intuitive. And it seems a loss of time and trouble to indicate the character which separates these two from each other. It is not the same when we descend the scale of organisms. Then we arrive at an inferior region where the distinction disappears gradually, and we soon conceive the existence of a frontier zone between the animal and vegetable, a neutral territory which has been designated the kingdom of Protista. They reproach naturalists for admitting the kingdom of Protista, accusing them of doubling the difficulty, instead of abolishing it; since it is necessary to establish a distinction between Protista, on the one part animal, on the other vegetable. That objection could be made every time they established a new division in the organic kingdom. It does not signify anything for those who know that all divisions that trench on biology are purely subjective and that nature does not bend to our strict system of classification. Natura non facit saltus. All living bodies can be decomposed into visible elements under the microscope, and these have been named Plastides or Cells. That word is employed in a more general sense. The most simple Plastide is the Cytode, a simple mass of protoplasm without a nucleus or membranous envelope. A cell in a restricted meaning of the word is a Cytode presenting a nucleus, that is to say, a mass of protoplasm in the midst of which is a distinct part of the substance ambient differentiated by its aspect and its property. 1. Plants and animals are always produced under the influence of a living body similar to themselves. 2. They develop from a germ or rudiment, and run through a course of changes, to a state of maturity. 3. Plants increase by a process through which foreign materials are taken, made to permeate their interior, and deposited interstitially among the particles of the previously existing substance; that is, they are nourished by food. 4. Plants and animals alone possess the power of assimilation, or the faculty of converting the proper foreign materials they receive into their own peculiar substance. 5. Connected with assimilation, as a part of the functions of nutrition, is a state of internal activity and unceasing change in living bodies; these constantly undergoing decomposition and recomposition, particles which have served their turn being continually thrown out of the system as new ones are brought in. This is true of both plants and animals, but more fully of the latter. 6. The duration of living beings is limited. They are developed, they reach maturity, they support themselves for a time, then perish by death sooner or later. Mineral bodies have no life to lose, and contain no internal principle of destruction. Once formed, they exist until destroyed by some external power. They lie passive under control of physical forces.