Age and area; a study in geographical distribution and origin of species . sdescended from species of larger area, so also we have seen thatthe monotypic genera are to be looked upon in general asdescended from larger genera. The way in which the numbers ofgenera, not only in the total, but family by family, are arranged(cf. p. 187) in hollow curves, with a great preponderance of mono-types and steady decrease to a few of large numbers, shows that ch. xxii] GEOGRAPHICAL DISTRIBUTION 241 there is a definite mechanical relationship between them. If weimagine existing genera to give rise to new g
Age and area; a study in geographical distribution and origin of species . sdescended from species of larger area, so also we have seen thatthe monotypic genera are to be looked upon in general asdescended from larger genera. The way in which the numbers ofgenera, not only in the total, but family by family, are arranged(cf. p. 187) in hollow curves, with a great preponderance of mono-types and steady decrease to a few of large numbers, shows that ch. xxii] GEOGRAPHICAL DISTRIBUTION 241 there is a definite mechanical relationship between them. If weimagine existing genera to give rise to new genera, as they giverise to new species, by mutations at intervals, we shall thenexpect that genera as a whole will follow the law of compoundinterest. But if this be the case, then it follows that whilst thenumber of genera plotted to the numbers of species that theycontain will give a hollow curve like those on p. 237, the loga-rithm of the number of genera plotted to the logarithm of thenumbers of species that they contain will give a straight line1. Number of species. 0 2 4 -6 -8 10 1-2 14 16 18 20 22 24 log (N9 of species) Logarithm curve for all Flowering Plants (from Willis, Dictionary).(By courtesy of the Editor of Nature.) That this is in fact very close to the actual truth when con-siderable numbers are dealt with is shown by the figures onpp. 241, 242, which give the logarithmic curves for all floweringplants, for the Rubiaceae, and for the Chrysomelid beetles. Thissubject must also be left for further consideration in a later it to say for the present that the evidence is decidedly infavour of the origin of new species and genera from old bymutation, which in the long run has followed a very definite 1 For this deduction I am indebted to my friend Mr G. Udny Yule,, 16 242 GEOGRAPHICAL DISTRIBUTION [pt. ii N2 of species 10 30 ioo
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