The final paragraph of The Origin of Species paints a picture of a tangled bank to which any biologist might warm. Is the spell broken by the counting of worms or reckoning of calories? I hope that some who thought that way can see now the pleasures and usefulness of biological arithmetic. I began this book by alluding to the accelerating rise of quantitative biology, and have made use of recent examples, but just a few pages on from the Preface I was quoting not the latest papers in cellular biology or genetics, but Darwin, and even Harvey. My concern, however, is very much with ways of thinking and ‘transferable skills’ (to use a current educational buzz-phrase) – and I think it interesting to see how the Ancients exercised them in contexts that are still meaningful. As to the actual biology, which of course I rank no lower, the plants and animals in the book ‘all interweave’ in consequence.