In fact, however, it is not so.

Each gemmule, according to Mr.Darwin, is really the seat of powers, elective affinities, and special tendencies as marked and mysterious as those possessed by the physiological unit of Mr.Spencer, with the single exception that the former has no tendency to build up the whole living, complex organism of which it forms a part.

Some may think this an important distinction, but it can hardly be so, for Mr.Darwin considers that his gemmule has the innate power and tendency to build up and transform itself into the whole living, complex cell of which it forms a part; and the one tendency is, in principle, fully as difficult to understand, fully as mysterious, as is the other.

The difference is but one of degree, not of kind.

Moreover, the one mystery in the case of the "physiological unit" explains all, while with regard to the gemmule, as we have seen, it has to be supplemented by other powers and tendencies, each distinct, and each in itself inexplicable and profoundly mysterious.