136 TEST CASES [ch. xii 



work (16). All that we are at present contending for is that 

 species and genera were formed each at one mutation, and that 

 the change went downwards to the species, not upwards from it, 

 as required by natural selection (of course a genus cannot exist 

 without one species. 



In the same chapter (ix) we have also described the sub-family 

 Silenoideae of Caryophyllaceae, and shown that the first split in 

 the key throws Silene with 400 species to one side, Dianthus with 

 300 to the other. 



This phenomenon, which is so common that it must have a 

 reason behind it, occurs in a great number of cases. Picking up a 

 few copies of the Pflanzenreich as they come, the first is Maran- 

 taceae, where Calathea and Maranta, the two largest genera, are 

 separated by the first split. In Myrsinaceae, Ardisia goes one 

 side and Rapanea the other. In Amarantaceae, Alternanthera 

 goes one side, Ptilotus the other. In Cyperaceae Cyperus and 

 Carexdo the same; in Eriocaulaceae£'Wocat//on and Paepala?7thus. 

 In Hydrophyllaceae, Phacelia and Nama go one side, and Hy- 

 drolea, with only nineteen species but very wide distribution, the 

 other. In Monimiaceae (p. 33) Siparuna goes one side and 

 Mollinedia the other. And so on indefinitely. 



It is thus clear that as the position of the largest genera, and 

 their sharp distinction in the great majority of cases, agrees with 

 what is required by the diff'erentiation theory, while that of 

 natural selection can give no idea where they will be found 

 in a family, the evidence of this test is in favour of the former. 

 Inasmuch as the classification of animals is equally possible, with 

 equally good results, when conducted upon the same lines as that 

 of plants, and as it shows the same hollow curves, it would seem 

 highly probable that the same general principles have guided the 

 evolution that has gone on in them also. 



TEST CASE XXI. THE POSITION OF 

 THE LARGE FAMILIES 



We may even carr}^ the supposition outlined in the last test a 

 stage farther, and apply it to families, saying that the very large 

 ones will be very widely separated. We are still so undecided 

 about the proper classification of the larger groups of plants that 

 it will not do to push this very far, but one may note that the 

 three largest families of all, from the latest figures in my posses- 

 sion, are Compositae (18,039 species), Leguminosae (12,754) and 



