The early successes of genetics and plant breeding and the still earlier successes of microscopy and chromosome study led to disputes, which were aggravated by lack of understanding between languages, professions and techniques. But their primary source lay in each pioneer's insistence on a uniformity of his own natural law. Bateson's exclusion of nucleus and cytoplasm was followed (in 1926) by Morgan's exclusion merely of the cytoplasm. An anti-genetic and anti-evolutionary revival was favoured by these disputes and has left its traces with us today. The idea of a uniformity in heredity or the genetic system is once again an obstacle to understanding. For, in the practice of plant breeding, we are faced by a conflict between evidence on experimental and evolutionary time-scales. Louis de Vilmorin, Darwin and Mendel thought of this problem under the title of the 'causes of variability'. We can now recognize that the experimental or classical models of mutation and recombination of genes and chromosomes is no longer universally sufficient either for organisms or for their chromosomes. Variability in higher organisms seems to have a variety of pre-nuclear as well as nuclear foundations.