CHAPTER X 



GEOGRAPHICAL DISTRIBUTION 



112. Significance of Geographical Distribution. From 

 the evidence of comparative anatomy and comparative 

 life histories, and also from the geological record (to be 

 noted later), it has been possible to determine the course 

 of evolution, in broad outlines, with reference to certain 

 of the larger groups of plants. As noted above, we may 

 learn that, in all probability, ferns preceded gymnosperms, 

 and gymnosperms preceded angiosperms; but within these 

 various groups, and for living forms, the problem becomes 

 increasingly difficult. For example, how shall we deter- 

 mine whether the family represented by the bracken fern 

 (Polypodiacece) is more ancient or more modern than the 

 royal-fern family (Osmundacece)? Is the maiden-hair 

 tree (Ginkgo) a younger or an older species than the pine 

 and the hemlock? Did herbs precede trees in the evolu- 

 tion of Angiosperms, or vice versa? This question of the 

 relative ages of living groups is greatly illuminated by the 

 evidence afforded by the facts of geographical distribution 

 of fossil and living forms. 



Darwin spoke of geographical distribution as the 

 ''almost keystone of the laws of creation," 1 and one does 

 not need to pursue the study of that subject far to under- 

 stand the truth of his statement. Before the diffusion of 



1 The interested reader will wish to consult those two remarkable chapters 

 (XII and XIII) of volume two of the Origin of Species. 



139 



