The process of reproduction leads to rejuvenescence in the protozoa, and it might therefore be imagined that in the higher animals continued activity of the sexual glands would similarly exert a rejuvenating influence on the remaining cells of the body. But the process of specialization has gone so far in the cells of the higher animals that the highly differentiated cells, especially of the nervous system, cannot fairly be compared with the constituent molecules of a unicellular organism. As sexual activity is the biological reason for existence and as in vigorous men sexual power may last long beyond the usual period, man being regarded as old as his sexual glands, it has been widely imagined that the functional activity of the sex glands is in some way a cause, rather than a manifestation, of the preservation of bodily vigour, and conversely that failure and atrophy of the sexual glands cause old age and senility. There is an attraction, though not without the danger of fallacies, in interpreting the words of older writers as prophetic anticipations of quite modern knowledge. But with this caution in mind there is some ground for crediting Hufeland[93] with some idea of the internal secretion of the sexual glands: as long ago as the end of the eighteenth century he wrote “the organs of generation have the power of secreting the finest and most spiritual parts of our nourishment; but at the same time they are so organised that these perfected and ennobled juices can again return and be received into the blood. Like the brain, therefore, they belong to those important organs which serve for bringing to perfection and ennobling our organic matter and power, and even ourselves.”