I wonder whether Anthony van Leeuwenhoek would have considered me as an appropriate choice for this lecture. For the past six years my interests have been in the rather unmanageable field of the genetics of cultured human somatic cells. These cells are animalcules only because we make them so. I can therefore look at the genetics of micro-organisms as an outsider, admittedly not wholly dispassionate. This is a pleasant task, because if there is a field of biology which has made great, unexpected and illuminating advances since 1940, this is it. Genetic analysis up to that time consisted in deducing the genetic constitution (‘genotype’) of an individual, which underlies its relevant somatic characters (‘phenotype’), from the distribution of these characters among its ascendants and its descendants. It therefore required the analysis of the results of breeding— experimental or not—and was limited to organisms with sexual reproduction. It was particularly illuminating in those multicellular organisms in which there is a clear distinction between germ cells (‘gametes’) and soma. Here, short of a preformistic process, it is clear that we can distinguish between
determinants
of hereditary characters and the
characters
themselves.
