158 TEST CASES [ch. xiii 



Nepenthes has 60, all living in very much the same conditions. 

 Begonia, mostly in the undergrowth of damp forests, has 750, the 

 xerophytic Crassulaceae 25/1500. Impatiens, mostly in the moun- 

 tain flora of India and Ceylon, has 350 species. There are six 

 species of Sonneratia, all mangroves, whose conditions of life 

 must be the saine. And so on. From these one can work down- 

 wards through smaller and smaller families, showing less and less 

 variety, down to single species like Hipjyuris vulgaris of family 

 rank. The smaller the family, on the average, the smaller is the 

 area that it occupies (size and space, p. 113 of Age and Area). 



Perhaps the most striking example of a great number of species 

 all occupying practically identical conditions is the existence of 

 the great group of the Fungi, more especially those that are 

 internal parasites, where the conditions must be exactly the 

 same, except for the chemical differences in the sap of one host 

 and of another, differences which must be discontinuous, as the 

 different chemical substances that occur are discontinuous. The 

 eight genera Clavaria, Fonies, Marasmius, Miicor, Penicillium, 

 Peronospora, Puccinia and Saccharomyces, all living in extremely 

 uniform conditions, have 2500 species among them. 



As in related forms the number of species goes up with the age 

 and distribution of the genus or family, it is much simpler to 

 look upon it as going simply with the age — the larger genus or 

 family, with the larger distribution, is the older. If the conditions 

 also become more varied with increasing age of the family, as they 

 almost always do, this probably helps to increase the number of 

 species by the stimulus that it gives. There is nothing to be 

 extracted from the figures that will go to show that natural 

 selection, or variety of conditions, is responsible for the numbers 

 of forms that exist. Probably as time goes on, and at any rate if 

 there is any stimulus, evolution has to go on. 



TEST CASE XXX. A COMMON TYPE OF DISTRIBUTION 



IN INDIA AND ELSEWHERE 



A proposition very difficult of explanation is put before the sup- 

 porters of natural selection by what is a very common type of 

 distribution, long ago pointed out by Dr Guppy in the islands of 

 Polynesia, and by the writer in India, Ceylon, and elsewhere. 

 This is the polymorphous widely ranging species, accompanied 

 by few or many species confined each to one part only of its 

 range, and endemic to the regions that they occupy. Guppy noted 

 three stages in the development of local endemism. First, the 



