DERIVATION OF THE FLORA AND FAUNA 37I 



famous London clay from lower Eocene, where a wealth of fruits and seeds were 

 found. The corresponding fossil floras of California include an increasing number 

 of plants that cannot be distinguished from living genera, and abundant evidence 

 shows that since late Pliocene "new species of woody plants have been added to 

 the Californian flora only in such large and complex genera as Eriogonum, Ceanoihus, 

 Arctostaphylos, and various Conipositae'' (p. 521). The fossil floras of East Asia point 

 in the same direction, the rate of evolution on woody plants has, since the middle 

 of this period, been as slow or slower. 



The most likely inference on the basis of all available evidence is that most of the 

 woody species of to-day have existed for five million years or more, and that the evolution 

 of the genetic isolation mechanism separating them took place largely during the early 

 and middle parts of the Tertiary period (p. 522). 



With regard to the herbaceous floras there is evidence for a rather rapid evo- 

 lution during the later Tertiary. Stebbins quotes the results obtained by Elias 

 in fossil caryopses dating from lower Miocene to middle Pliocene and showing a 

 distinct progression until the most recent ones cannot, to judge from the illustrations 

 redrawn by Stebbins, be distinguished from Stipa or Piptochaetium, an evidence 

 from palaeontology which strengthens the theory of SiNNOT'r and Bailey already 

 referred to. 



Isolation as a cause of evolution. — There is no reason why a species, which 

 has migrated to an island, should change and give rise to new forms only because 

 it is spatially isolated. A form of a varying population which gets isolated on an 

 island, may possess characters making it stand out as a more or less well-marked 

 form. As Jordan {131, 132) says, isolation is not the direct cause of the origin. 

 Spatial relation, Stebbins remarks, 



may persist over long periods of time without causing the isolated populations to diverge 

 from each other enough to become recognizably distinct or even different. . . . the nature 

 of many distributions strongly suggests that some of these disjunct segments of the same 

 species have been isolated from each other for millions of years. There is some reason 

 for believing, therefore, that geographic isolation alone does not result even in the 

 formation of subspecies. ... In small populations, which are particularly frequent on 

 oceanic islands, spatial isolation is the usual precursor to divergence in non-adaptive 

 characters by means of genetic drift or random fixation (p. 197). 



Stebbins also points out that spatially isolated races or species usually are 

 separated also by ecological barriers, because different areas also differ ecologically; 

 distance may not be the original isolating factor, separation and differentiation may 

 be due to the selective effect of ecological and climatic factors before geographical 

 separation occurred. The Hawaiian Islands offer a wide field for a study of isolation 

 and segregation resulting in local endemism (^^^). The continental flora inherited 

 by a Great Hawaii included numerous populations gradually taking possession of 

 Ihe new soil, a process of very long duration, volcanic activity progressing from 

 west to east, and when separation took place, different forms and species became 

 isolated on different islands. The wonderful systems of valleys, effectively separated 

 by high and very steep ridges, furthermore promoted segregation. Little has been 



