We have said that the fundamental doctrine of the Pennsylvania system of prison discipline is embodied in one compound word--CONVICT-SEPARATION.--How can we conceive of a “_modification_” of this elementary principle? It must be adopted as a whole or rejected as a whole. The structure of the building, in which the separate principle is carried out, may be modified. The method of proceeding in the institution, as it respects instruction, whether in letters or trades, or as it respects privations, indulgencies and punishments may be modified. The cells, the occupations, the diet, the mode of heating and ventilating, &c., may all be modified, but the principle of separating the convicts one from the other, is, or it is not, the basis of the system. It is not susceptible of modification. If separation is the principle, it is one thing,--if association is the principle, it is another thing.--The _degree_ of association or of separation, is not involved. When, therefore, we are told of “a necessity for modifying, to a certain extent, the Pennsylvania system, by allowing a certain class of convicts to be associated” for any purpose, or for any period of time, the phraseology is open to misapprehension at least, if not to animadversion. The phrase “_modifying_ to a certain extent” is doubtless inadvertently used in such cases for the phrase “_abandoning_ to a certain extent.” Words are significant of ideas, and ambiguity in the former, necessarily leads to ambiguity in the latter. If we speak of the associate system, as “modified” by the occasional separation of a stubborn class of convicts, or of the separate system, as modified by the occasional association of an imbecile class, we soon confound association and separation, and actually have neither the one nor the other. We apprehend that this is the very position to which some of the opposers of convict-separation would not be unwilling to see it brought. Indeed, some who profess to be staunch advocates of separation, have conceded (inconsiderately, as we think) that each plan has its own advantages, and that the best system would be the product of a combination.