It is, of course, obvious that in this case the union of two organisms which have become too small may be of advantage in bringing them back to the requisite normal size; but this is an isolated special case, which certainly does not justify our regarding conjugation as a means whereby diminished bodily size may be brought back to its normal proportions. By far the greater number of unicellular organisms are not permanently diminished in size by division, and even in the Diatoms the mass of the two fused individuals does not amount to the normal size of the species, so that even in this case there must be growth subsequent to the conjugation before the normal is re-attained. It may be doubted, therefore, whether the increase in mass is, even in the case cited, the essential event in conjugation, and whether there are not other effects which we cannot clearly recognize. Here, too, there must be differences between the two conjugating individuals, as we have just seen, for if they only communicated something similar to each other, the result would be an increase only in their mass, not in their qualities. Although we cannot demonstrate differences of this kind in the case of the lowly organisms with which we are now dealing, we may assume their existence from analogy with the higher organisms. We know, especially through G. Jäger, that in Man every individual has a specific exhalation, his particular odour, and that in the secretions of his glands there are incalculably minute differences in chemical composition, which justify the conclusion that the living substance of the secreting cells themselves exhibits such differences, and that all the various kinds of cells in an individual are not absolutely identical with the corresponding cells of another individual, but that they are distinguished from them by minute yet constant chemical differences. The assumption that differences of this kind exist even in unicellulars, and in all lowly organisms generally, is not a merely fanciful one, but has much probability.