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. peduneulata, sessili- 

 flora, 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 ro- 

 bur. 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 palaeontol- 

 ogy, 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 



