GROWTH in the animal and vegetable kingdoms is a process from which no individual is exempt. Growth, that is increase in size and weight of the living matter forming the individual, results from assimilation. By assimilation, matter similar to that of the organism itself is built up from the foods consumed, which latter are moulded and converted to form the specific substance of which the whole individual is composed. Despite their general similarity of composition, both the living substance and the greater portion of the organic material in various organisms are different and distinctive, certainly for each particular species and even for each individual within any particular species.
The basic property of five matter is its power of building up substances similar to itself. Consequently, synthesis of living substances always presupposes the influence of some other five matter, and synthesis can only take place when such matter is present. Synthesis comes about in such a manner that fresh living matter makes its appearance complete with chemical, histological and anatomical formations identical to those of the organism entering directly and immediately into the assimilative process. In this identity between the products of assimilatory synthesis and the organized substances which are the instruments of the synthetic process, consideration must be paid to the specific assimilation applying to each organism, to each cell thereof, and even to each differentiable portion of such cells. Thus, the growth of an individual with all its chemical and morphological characteristics comes about through progressive assimilation and the number of individuals forming a species is likewise increased.
Assimilation is the process of manufacturing living matter proper, that is to say, the essential cell plasma, residing and living within the cell, whilst at the same time it produces inclusions, paraplastic materials, nutritional reserves or functional products playing a special part in metabolism and in the body functions. In short, through assimilation, nature is able simultaneously to create both the machine and its fuel.