As among unicellular Algæ it is frequently only gametes of different lineage which conjugate, so among animals and plants there are numerous cases in which the union of nearly related gametes is more or less strictly excluded, both by the prevention of self-fertilization (autogamy) in hermaphrodites, or by the prevention of inbreeding, that is, the continued pairing of near relatives. Now all the preventive measures which effect this are of a secondary nature; they are adaptations which result from the advantage involved in the mingling of unrelated germ-plasms, even though it sometimes seems as if they were an outcome of the primary nature of the germ-cells. The primary result of the mutual chemical influence of the two germ-cells upon one another is--apart from the impulse to development which the centrosphere of the sperm-cell supplies--as far as I see, only the more favourable or the more unfavourable mingling of the biophor- or determinant-variants, and the resulting increase or decrease in adaptive capacity, which leads to the better thriving of the offspring, or conversely to its degeneration. Everything else is secondary and depends upon adaptation, effected in very diverse ways, to secure the most favourable mingling of the germ-plasms for the particular species concerned. Undoubtedly the parental ids united through amphimixis have an effect upon each other, since throughout the building up of the organism of the child the homologous determinants struggle with one another for food, but they do not affect each other in the way that many prominent physiological and medical writers suppose, namely, that the union of the parental germ-plasms sets up a 'formative stimulus' which 'advances' or even 'greatly advances' the process of development in the egg. Parthenogenetic development goes on just as rapidly, sometimes even more rapidly than that of the fertilized ova of the same species! How can the supposed 'formative stimulus' be so entirely dispensed with in this case?