236 CALIFORNIA ACADEMY OF SCIENCES [Proc. 4th Ser. 



Of the above thirteen families of vascular plants, the Filices 

 contain the largest number of species ; and these, owing to the 

 small size of their spores, would obviously possess greater 

 opportunity of being disseminated over considerable stretches 

 of water than the plants of any other family in the list. Fur- 

 thermore the small number of endemic species of ferns leads 

 naturally to the supposition that there is a more or less con- 

 stant introduction of spores from the mainland, thus checking 

 any strong tendency for the species of ferns on the islands to 

 vary greatly from those on the mainland. This supposition is 

 supported by the fact that each collecting expedition brings to 

 light more continental species that were not previously known 

 to occur on the islands. 



While it is no doubt true that great changes in the biological 

 conditions must have taken place on the islands if there had 

 been sufficient subsidence to separate them from the mainland 

 by the depth of water that now exists, it is nevertheless not 

 likely that the changes thus brought about would have been 

 great enough to exterminate many families completely and to 

 reduce all others so greatly in number of genera and species 

 as is the case. Some genera and species would have probably 

 become extinct if there had been a great disturbance in the 

 biological conditions ; but at the present time most families are 

 represented by more genera on the mainland than species on 

 the islands. 



From the above facts there appears to be little evidence to 

 show that there has ever been a land connection between the 

 islands and the mainland, yet there is no very strong evidence 

 opposed to the view that the islands may have been connected 

 with each other, at some not distant geological period, either 

 as one large island or as two or three smaller ones. The rather 

 remarkable harmonic zoological relationships existing between 

 the different islands, as shown by Dr. Baur, are more easily 

 explained by supposing such a condition, than if each island 

 had been formed separately. The following table, which shows 

 the Pteridophytes and Spermatophytes common to the different 

 islands, lends support to this theory. 



