CHAPTER XVI 



ENDEMISM AND DISTRIBUTION: GENERA 



We have seen that endemic species are especially common 

 upon islands, upon mountain chains, and in more or less isolated 

 localities (small or large), and that in all such regions they in- 

 crease, on the whole, in passing from north to south, up to a 

 certain limit. We have also seen that it is probable that the great 

 bulk of them must be regarded as young beginners. But if this 

 be so, there is no logical reason why the same should not be true 

 of endemic genera, which occur in similar places, and there is 

 every probability in its favour. Of course, just as in the case of 

 species, there are doubtless many exceptions here and there, but 

 we are speaking of the genera in the bulk. 



When the number and proportion of endemic species is large, 

 there are generally to be found a fair number of endemic genera 

 also, but there seems no necessary relation between number of 

 species and number of genera; or perhaps rather, this relation 

 may be much interfered with by other causes. The Hawaiian 

 Islands have more endemic genera than Ceylon or New Zealand, 

 though they have many fewer endemic species ; on the other hand, 

 they are more isolated. This matter still requires more careful 

 investigation. 



The number of genera confined to islands or mountain chains 

 seems to increase with at least three factors — with the size of 

 the island or mountain chain, with the isolation of the same, and 

 with increased southern latitude, up to 45-50° S. The effects of 

 all these factors may be seen in the list below, by comparing, for 

 example, Ceylon and Java, Ceylon and the Hawaiian Islands, 

 and Ceylon and New Caledonia (which is much smaller). 



The greatest proportion of endemic genera to area is to be 

 found in some of the southern and comparatively isolated loca- 

 tions, e.g. in the islands of Juan Fernandez, the Mascarenes, or 

 NcAv Caledonia, in south-west South Africa, in parts of West 

 Australia, etc. But the actual numbers of endemic genera in- 

 crease with increasing area, as the rough figures^ on p. 170 show. 



As in the case of species, no country has all its genera endemic, 

 and most are very far indeed from this condition. Contrary to 



1 Taken, without criticism, from my Dictionary, and not revised in detail. 



