In the first place the vitalist asserts that mechanism is inadequate to explain the phenomena of metabolism, of transmission of nervous stimuli, or of development. It is upon the last of these that Driesch lays special stress.
He has urged, as we have seen, that although a mechanical explanation might be given (such an explanation has indeed been put forward by himself) of the specific differentiation of the organism by supposing the first-formed elementary organs, developed out of the substances given in the initial structure of the germ, to act and react upon one another in accordance with a certain harmony, provided for by the same structure; yet a mechanism which can be subdivided ad libitum or almost ad libitum, and each part of which will still give rise to a complete organism, is not to be conceived. The answer to this objection has, however, been supplied by the experiments of Driesch himself and of many others. For though it is true that each of the first two, four, eight, or even in some cases each of the first sixteen cells into which the fertilized ovum becomes segmented, can, when separated from its fellows, give rise to a complete organism, yet in all cases there comes a time when the parts cease to be totipotent and produce not whole but partial structures.
This invariable restriction of potentialities, which occurs earlier in some cases than in others, and is not due to mere deficiency of substance, is not hard to account for.
Those substances on the presence of which in the ovum, as experiment has taught us, the formation of the elementary organs of the embryo or larva depends, are arranged in different cases in different ways: and they certainly may be, and very frequently are, so distributed that while each of the first four cells contains a like quantity of each of these specific substances, arranged in it exactly as they were in the whole ovum, the next division will sunder these materials in such a way that of the resulting eight blastomeres four will have more of one of the primary egg-substances, less of another; the amounts apportioned to the other four being in just the inverse ratio of this: and the result will be a difference in the fate of the cells when they are isolated from one another. In those of the one group the proportions of the organs developed out of these substances will not be the same as they are in the other. This is precisely the result which experiment has revealed; it is exactly this result which Driesch has ignored, or rather attempted to explain away.
It is evident, then, that to some extent the parts of this mechanism are interchangeable, that it can be subdivided, and that each part, brought now under new conditions, will still possess the potentialities of the whole, just as such a mechanism as a rocket, out of which, under the appropriate stimulus, a certain pattern of stars is developed, might be subdivided into two or more rockets of half size or less. There is, however, a limit to this interchangeability, while if the subdivision be carried beyond a certain point the totipotence of the parts is lost.
If the number of these organ-forming substances given in the germ were very large, as large, let us suppose, as the total number of separately inheritable characters, it might indeed be difficult to imagine a mechanism divisible into even two totipotent parts. But from the need for this assumption we are saved by the second part of Driesch’s own Analytische Theorie, which accounts for subsequent processes of differentiation by attributing the production of new parts to the mutual interactions of those that are the first to appear. For this also experimental evidence, though meagre, is not lacking, while a close parallel is found in the dependence of certain bodily functions upon substances—the hormones of Professor Starling—secreted by other organs.
In the second place the vitalist maintains that the processes of metabolism defy, nay more, always will defy, chemical and physical analysis. The first part of this statement may be a true description of the knowledge of to-day, but the existence in the living body of the same elements as are met with elsewhere, the synthesis of complex organic substances, the establishment of the equivalence of the energy which leaves the body as mechanical work or heat to that which enters it in chemical form in the food, should surely make us hesitate before abandoning all hope of attaining to a chemistry of life.
And thirdly, there are physiologists who believe that the complex phenomena presented to us in the activities of the nervous system are susceptible of a purely mechanical explanation.
‘A feature’, says Gotch, ‘which more particularly suggests spontaneous cellular activity is the well-known fact that centrifugal discharges may continue after the obvious centripetal ones have ceased. This is pre-eminently the case when the central mass is rendered extremely unstable by certain chemical compounds, such as strychnine, &c. There are, however, suggestive indications in connexion with such persistent discharges. The more completely all the centripetal paths are blocked by severance and other means, the less perceptible is such persistent discharge, and since nervous impulses are continually streaming into the central mass from all parts, even from those in apparent repose, it would seem that could we completely isolate nerve-cells, their discharge would probably altogether cease.’ Even in the hyper-excitable condition produced by strychnine the spinal motor nerve cells do not discharge centrifugal impulses when cut off from the centripetal connexions. The physiologist, therefore, has ‘definite grounds for believing that, as far as present knowledge goes, both the production and cessation of central nervous discharges are the expression of propagated changes and that these changes reveal themselves as physico-chemical alterations of an electrolytic character. The nervous process, which rightly seems to us so recondite, does not, in the light of this conception, owe its physiological mystery to a new form of energy, but to the circumstance that a mode of energy displayed in the non-living world occurs in colloidal electrolytic structures of great chemical complexity.’
To all these considerations we must add the fact that life did once originate upon this planet from matter which was not alive, and that even now some inorganic phenomena present at least remote analogies with certain vital processes. Such are the structure, the spontaneous division, and the regeneration of crystals.