586 STRUCTURE AND LIFE HISTORIES 
and covered over with sediment, then elevated again as 
dry land, so that subsequent excavations have revealed 
the fossilized trunks and stumps (Figs. 413 and 4 14). Thus 
it is seen that, by a study of fossils, we may not only 
learn of their structure and thus fill in many of the gaps 
in the evolutionary sequence left by a study of forms now 
living, but we may also learn of the distribution of plants 
and animals in previous geological ages in other words, 
we have the basis for a science of fossil geography or 
paleogeography. 
610. Plant Migrations. With the development of 
Paleogeography, a clearer conception of the location and 
changes of the continental areas of the past is gradually 
being gained. As a consequence, plant geography is a sub- 
ject of increasing interest to the paleobotanist. More- 
over, geology, the fossil record, and the present zonal 
grouping of plants indicate that, in the past, the polar 
areas, once tropic or sub-tropic, must have been fruitful in 
new species. 1 High mountains or plateaus are also sug- 
gested as homes of plastic races. In the tropics environ- 
ments are more nearly static, and, it is reasonable to sup- 
pose, less likely to cause variation. It is known that, once 
established, many species move most readily along the 
geologic formation which supplies the exact soil constitu- 
ents, the rate of movement often being rapid. Flotation 
of seeds is also a factor. The facts here briefly cited rest 
on the observations of a large number of invesitgators, 
extending over more than a century. 
511. Distribution of Plants in Time. In addition to 
the distribution of plants in space (plant geography) , the 
1 Owing to the precession of the equinoxes these areas undergo an extreme 
variation in the length of winter and summer of 37 days every 12,934 years. 
