1 68 HEREDITY AND EVOLUTION IN PLANTS 



related to our modern elms. This genus comprises 

 only four living species, which occur in only three widely 

 separated areas, namely, the far East (Eastern China 

 and Japan), the area between the Black and the Caspian 

 Seas (Caucasia), and islands in the eastern Mediterranean 

 Sea. But a study of the fossil evidence shows that during 

 a preceding geological age this genus had a very extended 

 distribution, including central Europe, the Iberian penin- 

 sula, Iceland, southeast Greenland, Labrador, western 

 North America, and Alaska. Owing to profound changes 

 of climate, in the transition from one geological age to 

 another, Zelkowa was apparently unable to survive, 

 except in the two restricted and widely separated areas 

 where it is now found. 



122. Continuous Distribution. — Continuous distribution 

 is of two types: ubiquitous, like the bracken fern, and 

 isolated, like the redwoods. Sequoia. In the latter case 

 two suppositions are possible: either the species or genus 

 is very new and has not had time to spread {indigenous 

 endemic) ; or it is very old and a relict endemic, as defined 

 above. Which of these two alternatives is correct for 

 any given case may be ascertained only on the basis 

 of comparative anatomic evidence, or on fossil evidence, 

 or on both. 



The motile sperms and the structure of the wood of the 

 maiden-hair tree {Ginkgo biloba), for example, point 

 without question to affinities with an older type of seed- 

 bearing plants, the Cycads. In the case of the genus 

 Sequoia, with only two living species, the coast redwood 

 {S. sempervirens) and the giant redwood {S. gigantea), 

 restricted in range to one state, California, the fossil 

 evidence shows that these two species are the meager 



