Why Us?: How Science Rediscovered the Mystery of Ourselves. James Fanu Le

kangaroo from each other, and the tissues of which they are made – as readily as a grand palace is distinguished from a humble factory, and from the bricks and mortar of which they are constructed. But the elusive ‘form’ of polecat and squid, unlike that of the palace or the factory, has the further extraordinary property of remaining constant throughout their lives, even though the ‘bricks and mortar’ from which they are fashioned are being constantly replaced and renewed. From the first natural historian, Aristotle, onwards, it was presumed that some organising principle, some ‘formative impulse’, must both determine and ensure that constancy of form.

      The presiding genius of natural history, Baron Georges Cuvier (1769–1832), director of the Musée d’Histoire Naturelle in Paris, proposed two laws of that ‘formative impulse’, the laws of similarity (homology) and correlation. First, homology. Cuvier inferred from a detailed study of the ten thousand specimens in his collection that the diverse forms of animals each concealed an underlying ‘unity of type’, all being variations on the same ‘blueprint’: the wings of bird and bat, the paddle of a porpoise, the horse’s legs and the human forearm were all constructed from the same bones, adapted to their ‘way of life’ – whether flying or swimming, running or grasping.

      His second law, of ‘correlation’, asserted that the various parts of every animal, its skull, limbs, teeth, etc., were all ‘of a piece’, all correlated together, being so fashioned as to fulfil its way of life. Thus a carnivore, such as a lion or hyena, would have limbs strong enough to grasp its victim and muscular enough for hunting, jaws sufficiently powerful and teeth sharp enough to rip its flesh, and so on. ‘Every organised being forms a whole, a unique and perfect system, the parts of which mutually correspond and concur in the same definitive action,’ he wrote.

      Cuvier maintained that these laws dictating the harmony of the parts of the ‘unique and perfect system’ were as precise as those of mathematics. He could not specify the biological forces behind them, but they were not merely some theoretical inference. Rather, they could be ‘put to the test’, allowing him, to the astonishment of all, to ‘restore to life’ those fantastical and long-extinct creatures from long ago, reconstructing from the assorted bones and teeth of their fossilised remains a ‘megatherium’, or ‘huge beast’, a creature resembling a giant sloth which would stand on two legs to graze on leaves. ‘Is not Cuvier the great poet of our era?’ enquired the novelist Honoré de Balzac. ‘Our immortal naturalist has reconstructed past worlds from a few bleached bones … discovered a Giant population from the footprints of a mammoth.’