208 University of California Publications i)i Botany [Vol. 6 



Floral Relationships of Northern Islands 



From the above table it is seen that in the majority of instances the 

 islands of the northern group have a larger percentage of their floras common 

 with the islands of the southern group than with each other. 



These computations by Dr. Stewart suffer from the same defect 

 as my Table II. Abingdon and Albemarle have ninety-nine forms in 

 common ; this joint element forms 83 per cent of the flora of Abingdon, 

 it is true, but only 30 per cent of that of Albemarle. Furthermore, 

 his table ignores the all-important factor of size of the floras. Abing- 

 don, indeed, is only half as distant from Tower as is Albemarle; but 

 this does not raise any presumption in favor of a higher percentage 

 of common forms for Abingdon, as soon as it is borne in mind that 

 the flora of Albemarle is nearly three times as rich as that of Abingdon. 

 When in view of this latter difference 73 per cent of Tower's flora is 

 found to occur on Abingdon and only 68 per cent on Albemarle, it 

 is clear that the distance between the islands, or other geographical 

 factors such as the oceanic currents which Dr. Stewart is discussing 

 in the passage cited, are of considerable influence in determining the 

 various island floras. If the geographical relations of Abingdon and 

 Albemarle to Tower were exactly alike, the infinitely richer flora of 

 Albemarle would certainly be more abundantly represented on mea- 

 gerly clothed little Tower than that of Abingdon. 



Table VI, then, gives Dr. Stewart's data with the islands arranged 

 in order of the richness of their floras. It is obvious from this at 

 once that in general the number of species common to any two islands 

 depends not so much on their location relative to each other, nor their 

 altitude, rainfall, or position in certain winds or currents, nor on any 

 as yet unknown or mysterious cause, but on the mere wealth and 

 variety of their plant lives. This is the all-important factor, beside 

 which every other is comparatively insignificant. When this element 

 is considered, the internal floral relations of the Galapagos are sub- 

 stantially accounted for; when it is disregarded, they become unin- 

 telligible. The general regularity of the decreasing series from left 



