We have seen that the term "biological realm" can be applied to hierarchic development starting from the N C-N C radicals. We have employed this term for didactic convenience although it is unrelated to the commonly accepted concept of life. The study of hierarchic organization also led us to consider a concept of life and death which, while retaining some of the common meaning, also accords with the phenomena of hierarchic organization.

In the complex entity, each lower level entity lives and dies with relative independence. An organ can be dead and yet have living cells in it for a time. There are always dead cells to be found in living tissues and organs. It is the relative independence of the different hierarchic entities making up a complex entity that explains these seeming peculiarities.

Our concept of life and death stems from consideration of the nature of hierarchic entities. We have seen that all matter in nature, from the simplest to the most complex entity, is a result of heterotropy. The persistence of constants proper to each entity is distinctly opposed to homotropy. Life in its broadest sense, corresponds to the capacity of an entity to maintain heterotropy by conserving its characteristic constants. The life of any entity appears to be synonymous with conservation of its constants. An entity dies when it has permanently—that is, irremediably—lost its capacity to conserve the constants which characterize it. Death then represents exhaustion of heterotropy for the specific entity.

The fact that, in essence, life appears to be synonymous with the conservation of constants and is heterotropic, relates it, and especially its origin, to one of the important sources of heterotropic force, solar energy. A distinction has to be made between heterotropy as one of the fundamental laws of nature and the means by which it is exerted. Solar energy, with all of its quantas, would greatly increase the effects of heterotropic forces in nature. It would not create such forces but would simplify them and extend their applications. The origin of matter and, as we have seen above, of entities, biological or nonbiological, is in the final analysis the result of heterotropic forces. External conditions qualitatively and quantitatively influence operation of heterotropic forces.

The sun's heterotropic contributions have to be considered under this aspect. Through the quantas it disposes of, solar energy has not created life, as conceived above, but by permitting more and more entities to appear, has greatly facilitated their extension. Its effect, although certainly not limited to any group of entities, seems to be especially important to those forming the biological realm. Similarly, the effect, of a special type of energy, radiation, also must be considered. Radiation appears to be related to the elements, and will be discussed in a later chapter devoted to the elements.

Since life itself is related to changes directly aimed at conserving constants, in this broadest sense it is no longer limited to the specific group of entities found in the "biological" realm. Life has the same meaning for an atom, crystal or micelle, as for a cell, organ or organism. It is for this reason that knowledge of the mechanism through which constancy is achieved becomes of great importance in the study of all matter and especially of the biological realm.