The similarity between them is not the taxonomic key to some other, deeper, affinity, and our recognizing its existence marks the end, not the inauguration, of a train of thought.

In 'Herbert Spencer and the Law of General Evolution'. Spencer Lecture, Oxford, 1963: reprinted in Medawar, P. B. (1967). The Art of the Soluble. Methuen, London. Pp 37-58.