VARIATION AND DISTRIBUTION OF SPECIES. 137 



of a succession of forms through the deviation of anterior 

 forms is the most natural hypothesis, and the most accordant 

 with the known facts in palaeontology, geographical botany 

 and zoology, of anatomical structure and classification : but 

 direct proof of it is wanting, and moreover, if true, it must 

 have taken place very slowly ; so slowly indeed, that its 

 effects are discernible only after a lapse of time far longer 

 than our historic epoch." 



In contemplating the present state of the species of Cupu- 

 liferce in Europe, De Candolle comes to the conclusion that, 

 while the Beech is increasing, and extending its limits south- 

 ward and westward (at the expense of Gomferm and Birches), 

 the Common Oak, to some extent, and the Turkey Oak de- 

 cidedly, are diminishing and retreating, and this wholly ir- 

 respective of man's agency. This is inferred of the Turkey 

 Oak from the great gaps found in its j)resent geographical 

 area, which are otherwise inexplicable, and which he regards 

 as plain indications of a partial extinction. Community of 

 descent of all the individuals of species is of course implied 

 in these and all similar reasonings. 



An obvious result of such partial extinction is clearly 

 enough brought to view. The European Oaks (like the 

 American species) greatly tend to vary, — that is, they mani- 

 fest an active disposition to produce new forms. Every form 

 tends to become hereditary, and so to pass from the state of 

 mere variation to that of race ; and of these competing in- 

 cipient races some only will survive. Q\iercus Rohur offers 

 a familiar illustration of the manner in which one form may 

 in the course of time become separated into two or more 

 distinct ones. 



To Linnaeus this Common Oak of Europe was all of one 

 species. But of late years the greater number of European 

 botanists have regarded it as including three species, ^.jjef??<7i- 

 culata., Q. sessiliflora^ and Q. puhescens. De Candolle looks 

 with satisfaction to the independent conclusion which he 

 reached from a long and patient study of the forms (and 

 which Webb, Gay, Bentham and others had equally reached), 

 that the view of Linnaeus was correct, inasmuch as it goes to 



