SINNOTT: ISOLATION AND SPECIFIC CHANGE 



445 



vinced of the efficacy of selection, believe that each region has its 

 own characteristic environmental complex, different from that of all 

 others, which modifies directly the germ plasm of the animal and 

 plant types living under it and stamps upon them their local distinc- 

 tions. Both of these views regard the environment as the most 

 important factor in specific change and look upon isolation as the 

 agency which, through providing a comparatively simple and constant 

 environment, allows a much closer adjustment to it by the plant and 

 animal population than is possible on wider areas, and hence leads to 

 the production of large numbers of local species. Still another view 

 considers that most, if not all, of these endemic and peculiar forms 

 would have developed anyway under the progressive evolution of their 

 type, and owe their local character not to a dependence, direct or 

 indirect, upon a specific environment, but merely to the fact that 

 they have been unable to become dispersed abroad. 



An analysis of the insular floras under investigation presents certain 

 facts which have a bearing on the problem. It makes evident, in the 

 first place, that endemism is by no means uniformly characteristic of 

 all the elements in the flora but that it occurs very much more fre- 

 quently in certain of the great groups of vascular plants than in others. 

 The vascular cryptogams, for example, which comprise an important 

 part of the vegetation of these islands, include but few species or 

 genera which are confined to any one island or island group. The 

 glumaceous monocotyledons — Gramineae, Cyperaceae and Juncaceae 

 — ^which are also abundant, are represented infrequently among the 

 endemic forms, though they are somewhat commoner there than are 

 the vascular cryptogams. It is in the petaloideous monocotyledons 

 and the dicotyledons that the great bulk of the endemics occurs 

 throughout all of these insular floras. Not only hosts of the species 

 but almost all of the local genera belong to these groups. Certain 

 families, like the Orchidaceae and the Compositae, often contain 

 almost nothing but endemic species. How great is this disparity in 

 the extent to which endemism occurs is evident from the following 

 table, which is an average of the eight island groups investigated. 



What bearing have these facts on our problem of the origin of 

 local types? They offer little support, in the first place, to the theory 



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