Of course I am well aware that the two kinds of germ-cells have a strong attraction for each other, and that the protoplasm of the ovum actually exhibits tremulous movement when the spermatozoon penetrates through the micropyle. I myself observed this in the case of the lamprey (_Petromyzon_) when Calberla instituted his investigations on the fertilization of that animal, but has that anything to do with a formative stimulus? Is it anything more than the result of the chemotactic stimulus exerted by the substance of the ovum upon that of the spermatozoon and conversely? And have we any ground for seeing anything more in this than an adaptation of the sex-cells to the necessity of mutually finding each other out and thereafter combining? Two quite different things are often confused with one another in this connexion: the mutual attraction of the two kinds of sex-cells which tends to secure their union, and the results of this union. A more exact distinction is necessary between the effects and the advantages which allogamy brings in its train and the means by which it is secured in the different species. If amphimixis really set up a 'formative' stimulus, and if the amount of this was regulated by the differences between the two parental germ-plasms, then parthenogenesis, which implies the entire absence of the mingling of two parental cells, would necessarily be even less advantageous than amphimixis between near relatives; but this is not the case. Continued inbreeding leads in many cases to the degeneration of the descendants, and particularly to lessened fertility and even to complete sterility. Thus in my prolonged breeding experiments with white mice, which were later carried on by G. von Guaita, strict inbreeding, effected throughout twenty-nine generations, resulted in a gradually diminishing fertility, and similar observations have been made by Ritzema Bos and others. But why does not the same thing happen in pure parthenogenesis? My experiments in breeding parthenogenetic Ostracods (_Cypris reptans_) shows that these crustaceans, in the course of the eighty generations which I have observed till now[24], have lost nothing of their prolific fertility and vital power; and the same is true in free nature of the rose-gall wasp (_Rhodites rosæ_), which enjoys the greatest fertility notwithstanding its purely parthenogenetic reproduction, the females not infrequently laying a hundred eggs in a single bud. How does it happen that 'the mutual influence of two different hereditary substances which so powerfully promotes individual development' can be here altogether dispensed with? Only because it does not really exist, except in the imagination of my opponents, still influenced by the old dynamic theory of fertilization.