THE ORIGIN BY MUTATION OF THE ENDEMIC PLANTS OF CEYLON. 7 ( .t 



it is obvious that they can never have been much more numerous. 

 They must have evolved on the spot where they are found. Not- 

 withstanding this, they are well-marked Linnean species and ac- 

 cepted as such by Trimen and Hooker. Not rarely their distinguish- 

 ing characters are relatively large. 



In drawing such conclusions, however, even from a thorough 

 knowledge of a flora, one is often exposed to lay too much stress on 

 striking but exceptional instances, whereas it is only averages which 

 may really be relied upon. For this reason Willis has worked out 

 a method, which gives a large degree of accuracy and thereby affords 

 a firm and unattackable basis for his deductions. Trimen divides 

 all species into six classes: Very Common, Common, Rather Com- 

 mon, Rather Rare, Rare and Very Rare, and his estimates are 

 thoroughly reliable, as is shown by the clearness and regularity of 

 the results derived from them. 



In order to compare two or more groups of species Willis multi- 

 plies the number of species in them, belonging to each of these classes 

 by a factor indicating the degree of rarity according to the estimates 

 of Trimen. These factors are 1 for very common, 2 for common and 

 so on, up to 6 for very rare. In this way averages may be calculated, 

 which give the relative degree of rarity for any group under con- 

 sideration. 



Next, the plants of Ceylon are divided into three main groups, 

 one containing the endemic species, the second those confined to 

 Ceylon and Peninsular India, and the third the forms of wider 

 (although often not very much wider) distribution. In this way 

 Willis finds: 



No. of Species Rarity 



Mean rarity of all species 2,809 3,5 



Species of wide distribution 1,508 3,0 



Of Ceylon and Peninsular India 492 3,5 



Species endemic to Ceylon 809 4,3 



Species of all 23 endemic genera 52 4,5 



Species of Doona (endemic) 11 4,6 



Species of Stemonoporus (endemic) 15 5,4 



Thus the species of wide distribution are the commonest, those 

 of Ceylon and India have just the mean degree of rarity, but the 

 endemics are relatively rare, the rarest of all being the species of 

 the endemic genera and especially those of the only two genera 

 which are rich in endemics. Results of the same kind, obtained by 

 applying this method to different manners of bringing the species 



