The fundamental vital phenomena may be regarded as (1) nutrition and (2) reproduction. The conditions under which these activities may he manifested are narrow in general, and the more closely limited in respect to a particular organism in proportion as its organization is more or less complicated. These conditions be long to the environment, from which the organ ism must take the elements necessary to its continued repair of loss—in other words, get food. Essential elements are oxygen, water and certain salts; in a word, all the constituent ele ments of protoplasm, at the least, must be ob tainable from the air or water, in order that vitality may persist. It is needful, however, that certain harmful things (poisons) should be absent, or the organism will be killed. Death will also follow excess or deprivation of the proper proportions of the food elements; too much or too little water; a raising or lowering of the temperature above certain points; ex cess or defect of light, improper electrical con ditions, etc. The simplest animals and plants, consisting of little except protoplasm and water, have great power of resistance to untoward conditions, and the range of diversity in cir cumstances in which they are able to survive is very wide. Life in its higher manifestations
is dependent upon the maintenance of a few well-defined conditions, and that innumerable accidents may bring it to a permanent end (see DEATH), whereupon the material which it ani mated instantly becomes inert matter subject only to the laws that governing the inorganic world.
Reproduction is the means of securing that continuity of life which is one of its primal characteristics, and consists in the separation from the original organism of a part, relatively greater or less in proportion as the organiza tion so divided is simple or complicated, which part is equally endowed with the potentialities of living and reproduction, and normally will reproduce its kind in turn.
For an account of the speculations and rea soning as to the origin of life on the earth, see ORIGIN OF LIFE; PROTISTA ; MECHANISM AND VITALISM.