The so-called vital phenomena which take place in the textures of organisms are, for the most part, performed by the agency of the living cell elements, in which we can recognize independent manifestations of life, such as the response to stimuli, motion, nutrition, growth, etc. The living activity of organisms requires for its perfect development certain external conditions, namely, a certain degree of warmth and moisture. Without a certain degree of warmth and moisture the chemical interchanges just mentioned cannot go on, and the organism is either destroyed or remains in a state of inactivity.
The nutrition of the animal body which is accomplished by means of the processes of assimilation already mentioned enables it to grow, and, up to a certain point, increase in size, and further undergo many changes in form and texture. There is, however, a limit to this assimilative power: nutritive activity diminishes, growth gradually stops, and after a time decay appears and is followed by death.
Thus organisms exist only for a limited period of time, during which their size, form and functional activity are constantly undergoing some general alteration dependent on or concurrent with the incessant changes in their molecular construction.
This cycle of changes through which organisms pass we speak of as their lifetime. During this lifetime, at the period when their functional activity is at its height, they possess the remarkable faculty of producing individuals like themselves.
This is accomplished by setting apart a cell which, under favorable circumstances, assumes special powers of growth, increases in size by the rapid formation of new cells, and develops into an independent living unit. In time it arrives at maturity, and becomes like its parent, and then passes through the same cycle - by its power of assimilation it grows to maturity, reproduces its like, decays and dies.