192 HEREDITY AND EVOLUTION IN PLANTS 



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. 



134. 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, then much warmer than now, must have been fruit- 

 ful in new species. 1 High mountains or plateaus are also 

 suggested as homes of plastic races. 2 In the tropics en- 

 vironments are more nearly static, and, it is reasonable 

 to suppose, less likely to favor variation. It is knownthat 

 once established, many species move most readily along 

 the geologic formation which supplies the exact soil con- 

 stituents most favorable to their growth, the rate of 

 movement often being rapid. Flotation of seeds is also 

 a factor. The facts here briefly cited rest on the obser- 

 vations of a large number of investigators, extending over 

 more than a century. 



135. Distribution of Plants in Time. In addition to 

 the distribution of plants in space (plant geography), the 

 problem of their distribution in geologic time is one of 



1 Owing to the precession of the equinoxes these areas undergo an ex- 

 treme variation in the length of winter and summer of 37 days every, 1 2,934 

 years. 



2 Cf. pp. 148-149. 



