1893.] Cretaceous Plant Population. 341 
result, of the high variability that is experimentally noted in 
such an area. 
From the above, the explanation of the various peculiarities 
of the tension-line will readily be derived. Under the law of 
the struggle for existence, it will be seen that this struggle may 
be between individual and individual, individual and forma- 
tion, and formation and formation. All three phases are 
developed at the tension-line and since the causes of evolu- 
tion are working at their highest along this area, it happens 
that the results are numerically most apparent. The three 
phases of the struggle are not elsewhere so vividly brought to 
one’s notice, for at least the third will be wanting or felt but 
indirectly. It appears, as a result of the examination, that the 
character of the tension-line, as made out by observation, is 
capable of illumination by experiment and of explanation by 
the general and well-known laws of evolution. 
It is now my intention to show that the metaspermic plant- 
population of the Cretaceous, and especially of the Upper 
Cretaceous was a tension-line population. If this be possible, 
we shall expect that population to manifest tension-line char- 
acteristics. 
In the first place it is necessary to gain some comprehension 
of the Cretaceous geography. I shall concern myself here only 
with the Upper Cretaceous of America. At the time of the 
deposition of the Cretaceous clays and sandstones, the North 
American continent was much more restricted than now. It 
extended little south of Illinois, and was cut by a great Med- 
iterranean Sea into an eastern, larger and shorter, and a west- 
tern, smaller and longer continent. The Mediterranean Sea 
extended from the south to the extreme north, and was doubt- 
less highly saline in character. The coasts of this sea were 
generally low—at least this is derived from the evidence of the 
flora, according to Lesquereaux—but, here and there, doubt- 
less varied by some rocky promontory or headland. The 
shores were marked by frequent bays and estuaries, and the 
rivers which flowed into the Cretaceous oceans were probably 
tide-streams to a marked degree. The general character of 
the country was low or morassic, but this was certainly broken 
