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MAN IN THE MODERN WORLD shown that the smaller the island, and therefore the smaller its lizard population, the more different this has become from the mainland type from which it was originally derived (see Table). DIFFERENTIATION IN ISLAND LIZARDS area (arbitrary 0-6 6-12 12-18 18-24 24~3° 30-36 depth (m) units) <o-5 *4 at 0-5-1 I I i-5 I, 2 I, 2 2, 2£, 2& 3 3 4 5-10 I IO-IOO o 2 IOO-IOOO O Table showing the influence of time and of size of population on the differentia- tion of island lizards from the mainland form. The depth is the maximum depth of water between the island and the mainland; as the islands have been formed by subsidence, the depth gives a measure of the time since isolation occurred. The area represents the area of the island, which is a measure of the population. The figures 1-4 in the chequer-board represent degrees of difference of the island forms from the mainland form. It will be seen that on the whole the longer the time of isolation and the smaller the size of the population, the greater is the degree of divergence. (Reproduced by kind permission of the publishers of J. S, Huxley's Evolution : the Modern Synthesis, Messrs. Allen & Unwin.) The other chief method by which new species are formed is through genetic isolation. This happens when a new form, wholly or partly infertile when crossed with its parent, is produced by some genetic accident—by means of the reduplication of whole chromosome sets, 196