Chap. II.] DOUBTFUL SPECIES. 63 



limited, and that the doubtful species are in a feeble 

 minority. This seemed to be true, so long as a genus 

 was imperfectly known, and its species were founded 

 upon a few specimens, that is to say, were provisional. 

 Just as we come to know them better, intermediate 

 forms flow in, and doubts as to specific limits augment." 

 He also adds that it is the best known species which 

 present the greatest number of spontaneous varieties 

 and sub-varieties. Thus Quercus robur has twenty- 

 eight varieties, all of which, excepting six, are clustered 

 round three sub-species, namely, Q. pedunculata, 

 sessiliflora, and pubescens. The forms which connect 

 these three sub-species are comparatively rare ; and, as 

 Asa Gray again remarks, if these connecting forms 

 which are now rare, were to become wholly extinct, the 

 three sub-species would hold exactly the same relation 

 to each other, as do the four or five provisionally 

 admitted species which closely surround the typical 

 Quercus robur. Finally, De Candolle admits that out 

 of the 300 species, which will be enumerated in his 

 Prodromus as belonging to the oak family, at least two- 

 thirds are provisional species, that is, are not known 

 strictly to fulfil the definition above given of a true 

 species. It should be added that De Candolle no 

 longer believes that species are immutable creations, 

 but concludes that the derivative theory is the most 

 natural one, " and the most accordant with the known 

 facts in palaeontology, geographical botany and zoology, 

 of anatomical structure and classification." 



When a young naturalist commences the study of a 

 group of organisms quite unknown to him, he is at first 

 much perplexed in determining what differences to 

 consider as specific, and what as varietal ; for he knows 



