43 II. UNCERTAINTY OF SPECIES. 



both kinds of change occur both in large and in small 

 genera. 



Number of Species at different dates. 



1843. 1847. 1851. 18.56. 



Kubus 24 



Salix 57 



Hieracium 19 



Galium 15 



Atriplex 10 



Epilobium 10 



Batrachium 4 



Arctium 2 



Ulmus ; 7 



Filago 3 



Cerastium 9 



Lastrea 6 



Quercus 3 



Thlaspi 3 



Urtica 4 



Sparganium 3 



Barbarea 4 



Polystichum 2 



Pyrethi-um 3 



Glyceria 2 3 3 3 



Facts such as are shown in the two lists, ought to con- 

 vince any reasoning man who may hitherto have imagined 

 Species to be things fixed and certain in nature, that 

 nevertheless down to the present time many of them are 

 far from being fixed and certain in books. "Where then 

 is the proof of their certainty and fixedness in nature ? 

 If in existence anywhere, in regard to the plants of 

 England, it ought to be found in the books of a diligent 

 Botanist, who has devoted his attention during many 

 years specially to the plants of this one country, compa- 

 ratively of small area ; who has travelled much over that 

 area, as a practical investigator ; and who has well 



