. The course of evolution by differentiation or divergent mutation rather than by selection. Evolution; Natural selection; Variation (Biology). 70 DIFFERENTIATION [CH. VIII A, as the oldest genus, should have the greatest area and the greatest number of species, B the second, C the third, Bb probably the fourth, and so on, but only on averages. Whilst in Ranuncu- laceae Ranunculus has 325 species to 250 in Clematis, one would hesitate, and rightly so, to say that the former was the older, when one remembers that it is herbaceous, and Clematis shrubby. As time goes on, it is clear that the rate
. The course of evolution by differentiation or divergent mutation rather than by selection. Evolution; Natural selection; Variation (Biology). 70 DIFFERENTIATION [CH. VIII A, as the oldest genus, should have the greatest area and the greatest number of species, B the second, C the third, Bb probably the fourth, and so on, but only on averages. Whilst in Ranuncu- laceae Ranunculus has 325 species to 250 in Clematis, one would hesitate, and rightly so, to say that the former was the older, when one remembers that it is herbaceous, and Clematis shrubby. As time goes on, it is clear that the rate at which new genera. Fig. 7. Evolution by differentiation. Each genus is supposed to survive the whole way along the line at right angles to its origin, A still survives at H, B at Be, and so on. In order to save complication, the lines to show the gro^^i:h Bb, Be, &c. are not shown. are formed will increase. Each genus will begin with one species, and after a time will form more, so that the few older genera of the family will contain the greatest numbers of species. The result will be (cf. 66, p. 185) the gradual formation of the familiar hollow curve already described, with a few large genera of dif- ferent sizes at the top, and many monospecific genera at the bottom, the numbers increasing from top to bottom at an accelerating rate. As there will rarely, upon this theory, be any appreciable adaptational difference between species or even genera, there will be little or no reason why the older ones should be killed out (as there is under natural selection), and so the increase in numbers will lead inevitablv to the hollow Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Willis, J. C. (John Christopher), 1868-1958. Cambridge [Eng. ] The University press; [New York, Macmillan]
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