SINNOTT: ISOLATION AND SPECIFIC CHANGE 447 



doubtless almost invariably self-fertilized. In the glumaceous mono- 

 cotyledons, although crossing is certainly not uncommon, it will 

 probably be agreed that self-fertilization is also the general rule. 

 In these two groups we have noted that local species and genera are 

 very rare. In the petaloideous monocotyledons and in the great 

 majority of dicotyledons, on the other hand, the flowers are attractive 

 to insects and cross-fertilization preponderates. As far as our knowl- 

 edge goes, there are few petaliferous species which are not at least 

 occasionally cross-pollinated. These facts are significant when we 

 remember that it is among such forms that local types are produced in 

 great abundance. In short, our analyses of these insular floras sup- 

 ports strongly the theory recently emphasized by Lotsy and others 

 that hybridization is the most important factor in the production of 

 new forms; self-fertilized types changing slowly, cross-fertilized ones, 

 rapidly. The unequal development of endemism in certain floral 

 elements, therefore, which neither the theory of selection or that of 

 the direct effect of the environment can well explain, is thus under- 

 standable as the result of differences in methods of reproduction, and 

 is quite consistent with the theory that the production of new forms 

 is due primarily to internal causes. 



The evidence brought forward by our study of isolated insular 

 floras therefore seems to indicate that isolation is not a factor in 

 evolution but simply in distribution ; that it cannot create an endemic 

 species but can merely keep a species endemic. 



Summary 



1. Isolated regions are characterized by possessing large numbers 

 of local, or endemic, species and genera. 



2. In the insular floras investigated, endemism is not equally 

 distributed among all plant groups, the local species and genera being 

 rare among vascular cryptogams and glumaceous monocotyledons but 

 very common among petaloideous monocotyledons and dicotyledons. 



3. This fact seems to indicate that the environment, whether 

 acting directly or by means of natural selection, has not been the 

 determining factor in the development of endemic forms. 



4. Those groups which are poor in endemics are predominantly 

 self-fertilized, those which are rich in endemics, predominantly cross- 

 fertilized; suggesting that hybridization has been the most potent 

 factor in the development of these new forms. 



5. Isolation is therefore regarded not as the factor which, by pro- 

 viding a simple, limited and peculiar environment, is responsible for 

 the origin of locally developed species and genera; but merely as the 

 factor which, by prohibiting dispersal, maintains the endemic char- 

 acter of local types which owe their origin to other causes. 



