142 HEREDITY AND EVOLUTION IN PLANTS 



first, adopted by Alphonse DeCandolle, but finally aban- 

 doned by him in his Gio graphic Botanique Raisonie (1855). 

 About the middle of the last century Agassiz was urging 

 his autochthonous^ hypothesis, namely, that each species 

 originated where it is now found (indigenous), covering 

 from the first as large an area as at present. This hy- 

 pothesis, if true, would, as Gray pointed out, " remove the 

 whole question out of the field of inductive science." 

 There would be no incentive to study the question of 

 geographical distribution, and little of value could result 

 from such an investigation. Both Schouw's and Agassiz's 

 ideas have long since been abandoned. It is no longer 

 considered a matter of hypothesis or theory, but of well 

 established fact, that most of the existing species are im- 

 measurably older than the present configuration of the 

 continents; in fact many genera and families of Angio- 

 sperms of the present land flora were clearly defined as 

 early as the Tertiary period, and have undergone little 

 change since that remote time. 



113. Means of Dispersal. — The question of the means 

 of dispersal of the seeds and spores of plants is a large one, 

 and a voluminous literature exists on the subject. This 

 is not the place to go extensively into the matter, but a dis- 



ing that a. change of climate or soil produces a corresponding change in 

 the character of its plants; or that some casual difierence in the character 

 of the type of any given plant, may have become permanent by its being 

 isolated. It is in this way that constant varieties have arisen, which may 

 sometimes even have become real species, but on all these occasions it is 

 culture that has been the cause; as far as I know, we possess no facts to 

 prove that natural causes have produced this effect." Schouw also 

 reached the erroneous conclusion that the present flora was probably not 

 derived from the plants of preceding geological periods. 



' Autochthon, from the Greek afirA j, self -t-x^uK land, meaning /row the 

 land itself. 



