_Distinction between Active Protoplasm and Reserve-stuff._—If it is not exactly in these terms that Claude Bernard formulated this fruitful idea, it is at any rate in this way that it is to be interpreted. This can be done by giving it a little more precision. We apply more rigorously than that great physiologist the distinction drawn by himself between _really active and living protoplasm_ and the _reserve-stuff_ which it prepares. To the latter is restricted the destruction by the functional activity and the building up by repose. The classification of Claude Bernard is strictly true for reserve-stuff. It is easy to criticize the wavering and, as it were, dimly groping expressions in which the celebrated physiologist has shrouded his ideas. The old adage will excuse him: _Obscuritate rerum verba obscurantur_. In the depths of his ignorance he had a flash of genius; perhaps he did not find the definitive and, as it were, clearly-cut formula defining what was in his mind. But, in this respect, he has left his successors an easy task. _The Law of Functional Assimilation._—The progress of physiological knowledge compels us therefore to distinguish in the constitution of anatomical elements two parts—the materials of _reserve-stuff_ and the _really active_ and _living protoplasm_. We have just seen how the reserve-stuff behaves, alternately destroyed by functional activity, and built up afterwards by the ingestion of food, followed by the operations of digestion, elaboration, and assimilation. It remains to ask how this really living and protoplasmic matter behaves. Does it follow the same law? Is it destroyed during the functional activity, and is it afterwards replaced? As to this we can express no opinion. M. le Dantec fills a gap in our knowledge, in this respect, by an hypothesis. He assumes that this essentially active matter grows during functional activity, and is destroyed during repose. This is what he calls the _law of functional assimilation_. The protoplasm would therefore behave in an exactly contrary manner to the reserve-stuff. It will be its counterpart. But this is only an hypothesis which, in the present state of our knowledge, cannot be verified by experiment We are at liberty to assert either that the protoplasm increases by functional activity or that it is destroyed. Neither the arguments nor the objections pro or con have any decisive value. The facts alleged on either side are capable of too many interpretations.[10]