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 1s 
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. 
