That the molecule-individuals [of a same species] from the inorganic world are entirely identical to each other is, by the way, only true insofar our, perhaps much too crude, methods do not uncover any differences. Probably also these inorganic molecules [of the same species] are only functionally the same. Indeed, our methodical determination refers to the functional [aspect] anyway [The identity of a molecule, i.e. what sort of molecule a given molecule is, is in these methods determined by its reactive properties, i.e. by its behavior in the presence of other molecules.].
Approaching, on the other hand, things from the existential-functional aspect, then the precise organismic molecular weight has negligible significance, as a result of increase [and thus of variation anyhow] of the molecular weight through security-enhancing metamerization [formation of "reserve parts" (additional parts) of the body], through backup materials, through medium-adnexes, and many more. The organismic existential function [of the molecule] is only connected with molecular weight within a frame of magnitude, it is not strictly connected with a particular molecular weight. In the case of very small molecules things are different : For instance, an urea molecule ( CO(NH2)2 ) must have a molecular weight of 60, because already with 59 or 61 it cannot (not considering isotopes) be urea at all. Only 2 N's, 4 H's, 1 C and 1 oxygen connected in a particular way, result in urea, and then the molecular weight is definitely determined. For urea the molecular weight is not the only criterium (Ammonium cyanate and -isocyanate have the same molecular weight), but necessarily belonging to its characteristics. In higher organismic molecules with billions of atoms the precise molecular weight practically and theoretically is of no significance. Here one must rely on other criteria, chiefly that of an uninterrupted bonding-continuum, which holds for the smallest as well as for the largest molecules. As criterium it far surpasses that of constant molecular weight anyhow (which latter is very practical by reason of its easy experimental determination) as to its "essential" significance. The same goes for the "total identity" without "individual difference" [as alleged criterium of something to be a molecule]. Existentional-functionally, in vast aggregates [large molecules] the first one [pertinence of quality] is not demanded, and the second [individuals qualitatively the same] not found important, because it doesn't influence the essential [i.e. the common whatness of every such aggregate (its being a large molecule)].