238 GEOGRAPHICAL DISTRIBUTION [pt. ii 



40, and with more genera than ten of 73. The numbers increase 

 regularly with the number of genera. 



One may even find, here as elsewhere, that (as a general rule) 

 the small famihes, which, as already explained under Size and 

 Space in Chapter xii, will tend to be the latest arrivals, have 

 fewer species per genus. While the families of one genus in 

 Britain have 2-2 species per genus, those with more than one 

 genus have a generic average of 3-3 species. If one take New 

 Zealand, one finds the 34 families of one genus to average 2-8 

 species per genus, those with more 4-3. 



One may even take the families of one genus in a country, and 

 find that they are arranged in arithmetical order. In Britain 

 there are 20 of these with one species, 7 with two, and six more 

 with larger numbers. In New Zealand there are 18/1, 6/2, 3/3, 

 and seven more. And this rule appears to hold e%'erywhere. If 

 one take the British families of two genera, one finds 12 genera 

 with one species, 7 with two, 3 with three, and 12 others. In 

 Ceylon the bi-generic families show 26/1, 8/2, 3/3, 2/4, 1/5, 1/6 

 and 9, 19, and 20 species. Everywhere the arrangement of genera 

 by species follows this simple arithmetical rule, forming hollow 

 curves. Even the proportions of families and genera of different 

 sizes in a country show some resemblance. In Britain 35 per cent, 

 of the families are monogeneric, in New Zealand 37 per cent., in 

 Ceylon 44 per cent. In larger and less isolated areas the pro- 

 portions are smaller, and in the world they are only IS per cent. 



Another matter upon which it becomes needful to adopt a 

 somewhat different view-point is the Struggle for Existence. 

 We have seen that it can no longer be regarded as an important 

 determining cause in evolution, and that it is most strenuous for 

 the individuals of new species that are just commencing. If 

 they cannot succeed in this first struggle, they will simply die 

 out and leave no trace, but if they do succeed, they may be 

 looked upon as having passed through the sieve of natural 

 selection, and being, so to speak, certified as fit for existence in 

 the region where they arose. Until they have spread to some little 

 distance, however, they can hardly be looked upon as established, 

 for they M'ill be very liable to sudden extermination, whether 

 ideally or badly equipped for life. A fire on the tiny summit of 

 Nillowe-kanda in Ceylon (p. 55), for example, would probably 

 exterminate the three species that are confined to it (and cf. 

 Didymocarpus and Christisonia on p. 151). Once established on 



