. Heredity and evolution in plants . as dennedabove. Which of these two alternatives is correct forany given case may be ascertained only on the basisof comparative anatomic evidence, or on fossil evidence,or on both. The motile sperms and the structure of the wood of themaiden-hair tree (Ginkgo biloba), for example, pointwithout question to affinities with an older type of seed-bearing plants, the Cycads. In the case of the genusSequoia, with only two living species, the coast redwood(S. sempervirens) and the giant redwood (5. gigantea],restricted in range to one state, California, the fossil
. Heredity and evolution in plants . as dennedabove. Which of these two alternatives is correct forany given case may be ascertained only on the basisof comparative anatomic evidence, or on fossil evidence,or on both. The motile sperms and the structure of the wood of themaiden-hair tree (Ginkgo biloba), for example, pointwithout question to affinities with an older type of seed-bearing plants, the Cycads. In the case of the genusSequoia, with only two living species, the coast redwood(S. sempervirens) and the giant redwood (5. gigantea],restricted in range to one state, California, the fossilevidence shows that these two species are the meager GEOGRAPHICAL DISTRIBUTION 169 remains (relict endemics) of a genus of several species,which, in Tertiary times, was widespread over most ofthe northern hemisphere (Fig. 77). By a like balancing of evidence we are able to ascertainthat the ubiquitous fern family, Polypodiacece, withsome 200 genera and about 3,000 species, is a com-paratively modern group, while the Osmunda family,. FIG. 77.—Map showing the known geographical distribution of Se-quoia during the Cenozoic era. The cross indicates the only known loca-tion of living specimens. (After E. \V. Berry.) with only two (or possibly three) living genera and someten species, and with wide but discontinuous distribution,is much older. The greater antiquity indicated for theOsmundaceae by the facts of their geographical distribu-tion is also attested by fossil evidence, and further by thenature of their spores. The spores when mature containchlorophyll, and this fact, of itself, indicates antiquity;for this and other structural and physiological reasons, 170 III KKD1TY AND EVOLUTION IN PLANTS they quickly perish unless they find at once suitableconditions for germination and development. Thus theycould not spread rapidly over large areas. In the lightof these facts the only logical inference is that theirwide and discontinuous distribution must have requireda vast period of
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