340 The American Naturalist. [April, 
origin and modification of metaspermic species—although not 
generally recognized. It will be observed that while this ejec- 
tion is purely biological, it nevertheless bears comparison with 
certain physical phenomena as when, for example, the water 
contained between the pores of adjacent solids is squeezed to 
the surface of the mass, and is thus ejected from its original, 
more central position. : 
It appears from the preceding analysis of conditions certainly 
existing and verifiable, that two adjacent formations do not 
actually come in contact, but are separated by the thicker or 
thinner band of tension-line species. It further appears that 
the tension-line group is made up of, and constantly replen- 
ished by plants ejected from the interiors of both formations, 
and reaching the tension-line as it were, to use a metaphor 
from chemical science, “in the nascent condition.” This 
explains, or at least suggests the explanation of what other- 
wise would be difficult, viz., the large comparative number of 
species in the tension-line area, and the fact that higher types 
are aggregated there rather than in the solid portions of adjacent 
formations. For not only is the tension-area populated from the 
interiors of the adjacent formations, and thus comes to have a 
diversified group of inhabitants injected into it, but these 
inhabitants are likely to be the new forms, not in perfect har- 
mony with the general mass of the formation in which they 
originated—and therefore they are ejected. Furthermore 
plants reach the tension-area “on the move ”—their variations 
already begun and accentuated, it may be, by the forced jour- 
ney to the periphery of the parent formation. Therefore both 
the actual numbers of species in the tension-area being great, 
and these species being derived from widely different habitat 
and environment conditions, together with the high degree of 
plasticity acquired in the ejection and after establishment in 
the tension-area, it becomes evident that it is precisely at such 
a contact-region between two formations that conditions are 
highly favorable for specific and generic and eyen ordinal 
variations to set in. Thus the increased number of species in 
the tension-area is seen to be in part the cause, and in part the 
; 
i 
