I. Biological inheritance; genotype and phenotype It has o f course been clear from very early times that there is some

biological inheritance. The character o f an organism depends, to some extent at least, on that o f its parents. The problem o f how the parental influence is exerted was for long debated in purely philosophical terms, but with the invention o f adequate microscopes, and the discovery o f spermatozoa and o f the universal occurrence o f eggs, hypotheses o f a verifiable nature could be put forward. The first theory to gain general acceptance had a deceptive air o f simplicity; it was supposed that the sperm (or, for the feminists, the egg) contained the complete organism in miniature, which merely had to grow to become the new adult. Elaborate theories were evolved as to how these homunculi in their turn mated and reproduced; but still more efficient microscopes soon made it clear that the eggs and sperm do not in fact contain miniature animals, and the whole elegant edifice o f theory had to be abandoned.1