CH. XII] C. TAXONOMIC 137 



Orchidaceae (10,088). Here, incidentally, is a case for the provisos 

 with which I hedged age and area, that one must never com- 

 pare anything but close relatives as regards age. To say that the 

 Compositae are older than the Leguminosae is obviously a state- 

 ment with nothing to back it. But the fact is there, that one 

 could not easily find greater divergence than is shown by these 

 three families, which incidentally contain nine out of twenty-nine 

 of the genera containing over 500 species each. If we go over the 

 first ten families in point of size, we find the fourth, Rubiaceae, 

 to have one of these genera, the fifth, Gramineae, one, the sixth, 

 Euphorbiaceae, three, the seventh, Melastomaceae, two, the 

 eighth, Labiatae, one genus, while there are none in the other 

 two. But these eight families contain seventeen out of the 

 twenty-nine of these big genera, and most of the rest are in large 

 families, though there are a few small ones which contain very 

 large genera, like Begoniaceae. 



The twenty-seven large families with over 1500 species are 

 Acanthaceae, Apocynaceae, Araceae, Asclepiadaceae, Bora- 

 ginaceae, Bromeliaceae, Caryophyllaceae, Compositae, Cruci- 

 ferae, Cyperaceae, Ericaceae, Euphorbiaceae, Gesneraceae, 

 Gramineae, Labiatae, Leguminosae, Liliaceae, Melastomaceae, 

 Myrtaceae, Orchidaceae, Palmaceae, Rosaceae, Rubiaceae, 

 Rutaceae, Scrophulariaceae, Solanaceae, Umbelliferae. It will 

 be seen at once how wide a range they cover in the classification, 

 in fact touching all important parts of it. 



The evidence of both these test cases is strongly in favour of 

 divergent mutation, forming the whole family or genus at one 

 step. 



TEST CASE XXII. DIVERGENCE OF VARIATION. 



SYSTEMATIC KEYS 



In making keys to families or genera, by whose aid one may 

 determine the relationships and position of the plants with which 

 one is dealing, the taxonomist is concerned with providing the 

 easiest and most certain method of so doing. And it is a very 

 remarkable fact, that has hardly been sufficiently recognised, 

 that it is usually possible, without any very great difficulty, to 

 make a dichotomous key (sometimes trichotomous at certain 

 points), beginning at the top with characters that will separate 

 one sub-family from another, and working right down through 

 tribe, genus, and species, to variety, in the same way. This fact, 

 which upon the theory of differentiation must occur, does not 



