OF TAXODIUM DISTICHUM AND RELATED SPECIES 397 
as far north as Greenland in Miocene times. With the advance 
of the ice-sheet during the Glacial period it was probably driven 
southward into Mexico, where the same species or a closely related 
one exists to-day. In the meanwhile the deposition of the Lafay- 
ette formation had taken place, and with the retreat of the ice our 
Taxodium doubtless migrated northward and eastward until it 
occupied its present territory. It was probably during this migra- 
tion over the newly deposited Lafayette that the differentiation into 
two species took place, and 7. ¢mbricarium was evolved. 
Some of the authors quoted at the beginning of this paper 
have mentioned the occurrence of forms intermediate between 7. 
distichum and T. imbricarium. These might be of two kinds: 
first, individuals bearing branchlets of both kinds, and second, in- 
dividuals all of whose leaves are intermediate in form and position. 
The first case has been already discussed, and seems to pre- 
sent no difficulty. Individuals of the second kind also exist, and 
may perhaps even represent a distinct species. Such an interme- 
diate form is probably what Brongniart * described as Zaxodium 
microphyllum, and Parlatore + as 7. distichum 8 microphyllum, so 
that names are not lacking for it. The only locality cited by Par- 
latore for his variety is Louisiana, while the typical T. imbricarium 
is not known west of Alabama. This may be an indication of the 
progressive differentiation of the species as the genus migrated 
eastward along the Gulf coast after the Glacial period. 
In Georgia I have seen this intermediate form only in and near 
thé Ogeechee River, at Millen, Ogeechee, Rocky Ford and Mel- 
drim,. What the relations of the Lafayette and Columbia forma- 
tions are at these points I have not yet determined. 
There is another supposed species of TZaxodium in Mexico, T. 
mucronatum Tenore, of which comparatively little is known. 
Most of the accessible information about it has been summed up 
by Professor Sargent,t who is inclined to believe that “it may 
Prove to be a mere geographical form of our tree.” Professor 
Bray, in writing to me of the range of 7. distichum in Texas, 
€xpresses the opinion that this species as it passes into Mexico 
* Ann. Sci. Nat. 36: 182. 1833. 
TDC. Prodr. 16: 441. 1868. 
{Sylva N. A. 10: 150. 1896. Gard. & For. 10: 451. July 17, 1897. 
