396 HarpPeR: FACTORS INFLUENCING DISTRIBUTION 
stance, both species seem to grow only over calcareous substrata. 
T. distichum is almost entirely restricted to the Cretaceous and 
later formations of the coastal plain, but if Dr. Mohr’s report of 
its occurrence on the Tennessee River in Alabama is correct, it 
there extends beyond the inner edge of the coastal plain into the 
Palaeozoic region. In Georgia it does not seem to extend even as 
far inland as the Cretaceous, but probably neither the topography 
nor the chemical composition of the rocks in the Cretaceous region 
of Georgia are adapted to either species of Zaxrodium, for the 
country is quite hilly, with few swamps or ponds, and the rocks 
are mostly argillaceous. 5 
The habitat of 7. imbricarium, so far as underlying formations 
are concerned, seems to be still more restricted than that of 7 
distichum, 1 do not know of its occurrence on strata older than 
Tertiary, and it may not extend farther inland than the Oligocene 
division of the Tertiary. 
The difference between 7. distichum and T. imbricarium may 
be of comparatively ancient origin, for fossils resembling both 
species are found in the Cretaceous and Tertiary strata of North 
America and Europe. Some of the fossil species of 7 wxodium with 
spreading ieaves are referred to our living species 7. déstichum, but 
those with appressed leaves are usually referred to the related 
genus Glyptostrobus, which seems to be represented among living 
plants only by a single Asiatic species. More will be said of this 
genus later. 
But if both of our species of Zarodium have existed since Cre- 
taceous times, the question will naturally arise, what were their 
respective habitats before the Lafayette period, and also during 
that period, when the present coastal plain was all submerged ? 
I am not prepared to answer this question completely, but would 
suggest as a partial solution that 7. iméricarium has been evolved 
from the ancestors of 7. distichum since the Lafayette period, and 
for this reason the two species are not yet as completely differen- 
tiated as are those of more ancient origin. The occurrence of 
distichous spreading leaves on the branchlets of the young shoots 
of T. imbricarium would seem to substantiate this view, for yours 
shoots are usually supposed to show ancestral characters. 
It seems to be a well-established fact that 7. distichum grew 
