180 
of old world species would thus result in the conditions of differ- 
ence as seen to-day between the old world and North America.” 
Returning again to the subject of “ Pressures and Tensions,” 
the conclusion is reached that, “ Under the positive equatorial 
pressure opposed by the negative polar pressures, a segregation of 
metaspermic plants would take place,” that is, the weaker or older 
types of plant life would gradually find themselves crowded out in 
the struggle for existence. Another result which the author 
emphasizes is that the weaker ones are forced to fight in the front, 
and as an example of this principle cites the line between forest 
and prairie in the Minnesota valley, where he says: “It is not the 
characteristic grass of the prairie that grows close up to the char- 
acteristic tree of the forest, but between the two there is a zone of 
plants not perfectly established in either forest or prairie. This 
transitional formation * * * is generally composed of species 
weaker than the characteristic plants of either formation.” 
Under the present climatological conditions the author sssumes, 
just why he does not make apparent, that the equatorial pressure 
is increasing, while that of the poles is decreasing, and that the 
line of tension is thus gradually progressing towards higher lati- 
tudes, although liable to local fluctuations due to physical and 
biological causes. In effect, this is putting into a somewhat dif- 
ferent shape the observation of recent investigators, that there is 
an apparent gradual northward movement of our flora, which has 
been ascribed to a continuous amelioration of climate since Glacial 
times. 
The influence of this pressure is also discussed in its relation 
to the specialization of the structure and habitat. Thus the least 
specialized plants, the aquatic, occupy the least specialized habitat, 
whilst amongst the most highly specialized, as for instance, the 
epiphytic orchids in the group of Monocotyledones, occupy the 
most highly specialized habitat. 
Secondary longitudinal tensions, which are lines due to local 
geography, and minor tensions, due to the influence of topography 
in limited localities, are also discussed. 
Under the heading “ Outlines of Metaspermic History in the 
Northern Hemisphere” may be found a brief resume of what is _ 
known in regard to the development of the higher forms of vege- — 
