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 414) . 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. 



510. 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.^ 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 



' Owing to the precession of tiie equinoxes tliese areas undergo an extreme 

 variation in tlie length of winter and summer of 35 days every 12,934 years. 



