Animals move and have special organs of lo comotion; few plants move, though some climb, and minute forms have thread-like processes or vibratile lashes (cilia) resembling the flagella of monads, and flowers open and shut; but these motions of the higher plants are purely me chanical and are not performed by special or gans controlled by nerves. The mode of repro duction of plants and animals, however, is fun damentally identical, and in this respect the two kingdoms unite more closely than in any other. Plants also, like animals, are formed of cells; the latter in the higher forms combine into tissues.
Physiological Distinctions and Resem As has been said, the bodies of the lowest plants and animals are plainly enough made up of protoplasm. The irritability, con tractibility, and metabolism of a plant-cell or a living, free unicellular plant do not differ from those of a unicellular animal (Protozoon) of the same morphological grade. The movements of the lowest alga, the sensitiveness of the leaves of the mimosa-tree, of the sun-dew and other insectivorous plants, are due to the samepri marry cause as the movements of animals of all grades, as the power of lifting one's arm is fundamentally due to the contractibility of the protoplasm forming the cells of the muscles.
Also the differences in metabolism are not fundamental, those molds which do not contain chlorophyll and bacteria performing the same metabolic functions as regards carbon dioxide as animals. Animals are also subject to the same general tropisms as plants; they are geo tropic, heliotropic, thermotropic, hydrotropic, chemotropic, etc. (See TROPISMS). To a much greater extent than formerly supposed even in sects so highly developed as ants are subject to the influences of the primary factors of growth, morphogenesis, and of the conduct of life, and the instincts of animals in general are more dependent on these agents, on external stimuli, than was previously thought to be the case.
Plants Fixed Organisms; Animals as a Rule While the lowest plants (Protophytes) are, as entire organisms, often motile, free-swimming, closely resembling mo nads, the higher or more specialized forms, com prising the great majority of the vegetable world, are fixed and have always remained so.
It is this fixed condition of life which, so to speak, has held the plant world in an iron grasp and kept it within its natural limits. On the other hand, animals as a rule are active, free to move, restless. Whenever animals, though born as free-swimming germs or larva, are con strained by change of circumstances to become attached or fixed to the sea-bottom or solid objects, they degenerate and become more and more subject to the influence during growth of those cosmic and physical forces, such as grav ity, light, air, currents of water, etc., which determine the shapes and morphology of plants. Fixed animals, like the polyps, sea-anemones, sponges, Polyzoa, etc., which lead a purely vege table life, tend to assume plant-like shapes. Even the echinoderms, as the fixed crinoids, are plant-like, hence their name, sea-lilies. It is freedom of motion, greater activity, which have led to the vastly more complex and higher types of life in animals, to the development of a nerv ous system, and to the origin of mind and intelligence.
Plants Not the Primitive Basis of Ani mal
As the lowest plants and animals are scarcely distinguishable, it is probable that plants and animals first appeared contempo raneously; and while plants are generally said to form the basis of animal life, this is only par tially true; a large number of fungi are de pendent on decaying animal matter ; and most of the Protozoa (q.v.) live on animal food, as do a large proportion of the higher animals. The two kingdoms supplement each other, are mutually dependent and probably appeared si multaneously in the beginning of things. It should be observed, however, that the animal kingdom greatly overtops the vegetable king dom, culminating in man. (See ZOOLOGY). Con sult