1888.] Evolution as illustrated by the Geogr. Distrib. of Plants. 287 



the pouter, thus assimilating it in this respect to the fantail, and he 



adds the words : 



" I presume that no one will doubt that all such analogous 

 variations are due to the several races of pigeons having 

 inherited from a common parent the same constitution and 

 tendency to variation, when acted on by similar unknown 

 influences." 

 And after mentioning other instances of analogous variation he 



goes on to add : 



*' According to the ordinary view of each species having been 

 independently created we should have to attribute this 

 similarity .... not to the vera causa of commu- 

 nity of descent and a consequent tendency to vary in a 

 like manner, but to three separate yet closely related acts of 

 creation." 



. Again on page 127 we find the passage : 



" The difficulty of distinguishing variable species is largely due 

 to the varieties, mocking as it were other species of the 

 same genus " . . . . 

 ^".Elut the best evidence of analogous variation is afforded by 

 parts or organs which are generally constant in character, 

 but which occasionally vary so as to resemble in some 

 degree the same part or organ in an allied species. I have 

 collected a long list of such cases." 



Now it is obvious that if the individuals of allied species shew 

 -this tendency to parallel variation such tendency must be far stronger 

 among individuals of the same species and still more among individuals 

 ..of the same variety, and accordingly on page 91 Darwin says : 



'* The tendency to variability is itself hereditary, consequently 



they " {i.e. the individuals belonging to varieties of a 



species) " will likewise tend to vary, and commonly in the 



same manner as did their parents." 



These passages seem to prove almost conclusively that Darwin 



held that varying species have a tendency to vary in particular lines, 



determined by those lines of variation which have evolved the species 



itself. 



^On the other hand, however, there are passages in which Darwin 

 ; seems explicitly to deny the existence of a general tendency to 

 simultaneous variation. 



