The core of biology is variation — of genes, phenotypes, species, ecosystems. This is a major difference from the so-called exact sciences. Carl Pantin (1968) has written of the latter, “physics and chemistry have been able to become exact and mature just because much of the wealth of natural phenomena is excluded from their study”. He goes on, “There is no need for the physicist as such to go to biology for data until in the last resort he has to take into account the fact that the observer is a living creature. I would call such sciences ‘restricted’. In contrast, biology and geology are ‘unrestricted’. Men of science devoted to these fields must be prepared to follow the analysis of their problems into every other kind of science. If they wish to advance their subject they cannot possibly say, “I will not burden my mind with chemistry, physics, or anything but my special interest”.