Leaving on one side, then, the natural origin of protoplasm, the conditions of which are unknown, we find that, as a matter of observation, every bit of living substance, the history of which has been traced, is a fragment detached from some other bit which behaved in the same way. This is the basal fact of the continuity of organic evolution. But such a detached fragment has the property of increasing by taking up from the environment more of those elementary materials from which it is itself compounded in subtle synthesis. Nay, further, every fragment of which we know the history is found to increase in such a way as to reach, in form, structure, and idiosyncracies of behaviour, the likeness of the organism--plant or animal--from which it was derived. In the higher plants and animals the separated fragments or cells are the ova and sperms, or their equivalents, which unite, with fusion or coalescence of their nuclear matter, and thus give rise to a new individual in the course of embryological development. Now, as we have already seen, much modern biological discussion centres round the question whether the detached reproductive fragment, ovum or sperm as the case may be, is derived from the whole body of the parent, by what Darwin termed pangenesis or in some other way, or only from germinal substance set apart in development for this end. And we have provisionally accepted the hypothesis that it is the direct descendant of other reproductive cells; and that, throughout a long ancestry, stretching back into the far past, there never occurs in the direct line of genealogical sequence, any highly differentiated cell, such as a gland-cell, muscle-cell, nerve-cell; never, with certain reservations into which we need not enter, is found the representative of any tissue save that to which the reproductive function is restricted. In technical phraseology, the continuity of organic evolution is due to the continuity of the germinal substance.