86 Prof. A. Gray on Species considered as to 



historical connexion between congeneric species. He accepts 

 and, by various considerations drawn from the geographical dis- 

 tribution of European Cupuliferce, fortifies the conclusion (long 

 ago arrived at by Edward Forbes) that the present species, and 

 even some of their varieties, date back to about the close of the 

 Tertiary epoch, since which time they have been subject to fre- 

 quent and great changes of habitation or limitation, but without 

 appreciable change of specific form or character, — that is, with- 

 out profounder changes than those within which a species, at 

 the present time, is known to vary. Moreover he is careful to 

 state that he is far from concluding that the time of the appear- 

 ance of a species in Europe at all indicates the time of its origin. 

 Looking back still further into the Tertiary epoch, of which the 

 vegetable remains indicate many analogous, but few, if any, 

 identical forms, he concludes, with Heer and others, that specific 

 changes of form, as well as changes of station, are to be pre- 

 sumed. And finally, that " the theory 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 Cupuliferce 

 in Europe, DeCandolle comes to the conclusion that, while the 

 Beech is increasing, and extending its limits southward and 

 westward (at the expense of CunifercB and Birches), the common 

 Oak, to some extent, and the Turkey Oak decidedly, are dimin- 

 ishing and retreating, — and this wholly irrespective of man's 

 agency. This is inferred of the Turkey Oak from the great gaps 

 found in its present geographical area, which are otherwise in- 

 explicable, and which he regards as plain indications of a partial 

 extinction. Community of descent of all the individuals of spe- 

 cies 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 spe- 

 cies) greatly tend to vary ; that is, they manifest an active dis- 

 position 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 incipient races some only 

 will survive. Quercus 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 spe- 

 cies. But of late years the greater number of European botanists 



