Organisms as Physical Bodies

In order to account for the difference which evidently exists between living and non-living bodies, the physicallythinking biologist has recourse to the demonstrable complexity of organic structure. This intricacy of constitution provides the basis for a corresponding complication of process. Hence, in general, if we wish to understand the activities of organisms, we must first determine their structures or anatomies. The activities should then be found to follow logically, just as do the motions of a machine, as a necessary consequence of the relations of parts. The "necessity" is that of fundamental physical laws, or their corollaries or combinations. This conception of the basis of organic activity is usually called "mechanistic."

We must be careful, however, not to give the adjective, "mechanistic," an unduly old-fashioned interpretation. When we compare organisms to machines, we do not mean that they have a rigid and metallic constitution. We do not imply that the principles of mechanics, alone, are sufficient to account for their behavior. Mechanics is only one subdivision of physical science, and at the present time it occupies a rather subordinate position. It is true that the principles of mechanics have many applications to organic activity, but in order to understand such activity we must also utilize the laws of chemistry, optics, electrostatics and dynamics and, in fact, of probably every known physical science. It is by no means beyond the pale of physical explanation that organisms should be fluid, plastic, impressible, spontaneous, and even seemingly quite unreliable in their behavior.

We know that the complexity of structure which is characteristic of organic beings begins even at the molecular level, as a consequence of the unusual variety and size of the molecules which can be formed by the element, carbon, in combination with hydrogen, oxygen and nitrogen. Living beings appear primarily to be expressions of this chemical versatility of the carbon atoms. Such compounds of carbon include many substances which are of the colloidal type, having very large molecules which are easily modified in exact form and properties under the influence of relatively slight external forces,--or even forces of their own constitution, acting slowly. Protoplasm, which is the classical concept of living substance, is an elaborate mixture of such colloidal materials. As a consequence of the process which we call evolution, certain forms of protoplasm have become established on this planet through their superior ability to survive the destructive action of external and of internal forces.

Their most potent means for resisting such destructive action consist in an ability constantly to reproduce themselves out of raw materials found in their environments. This reproductive capacity is readily explicable in physicochemical terms as a manifestation of a process known as autocatalysis. 106 The continued struggle of protoplasmic matter for existence, over a long period of time, has led to the establishment of a vast multitude of arrangements for accomplishing this result, these arrangements corresponding to different organic species. In general, the most successful scheme has been that of generating a vehicle, taking the form of a living organism, which may be more or less complex. In their more primitive exemplifications, organisms consist of single so-called biological cells; but the greatest success in the struggle against dissolution is evidently achieved through the formation of larger organisms which consist of an intricate structure of differentiated cell units. Such many-celled organisms can be arranged in order of their complexity, the most intricate of all being, of course, the human.

Now, if it is true that organisms are actually built up in accordance with the above sketch, exclusively of physical units, it should really be possible in the last analysis to account for all of their properties and activities by means of purely physical principles. Virtually all modern biologists believe firmly in this possibility; in fact, it is the primary tenet of their scientific faith. Nevertheless, they are not so foolish as to suppose that their knowledge of the details of organic structure, and of the physics of such structures; is adequate to permit any very complete explanation at the present time. We are still in the process of investigating these things. Consequently, in a great many cases, we have to make use of principles which we cannot trace back in full detail to doctrines of ultimate physics. Moreover, if we are trying to develop a tentative explanatory scheme, as in the present book, we are compelled to fill in many gaps by the method of scientific guesswork or hypothesis. Just because he recognizes these facts, the physical biologist is not liable to be over-awed by vitalistic attempts to demonstrate the impossibility of physical explanations of certain kinds of organic behavior. It is not at all surprising that organisms exhibit very peculiar properties, since their structures are also very special; and physical explanations rest upon particularities of structure even more than they do upon the rigidity of ultimate scientific principles.