228 baker's north YORKSHIRE. 



in what are called critical genera, such as Rosa, Rubus, Hieracium, 

 Salix, and Viola, a series of forms which one author will include 

 under a single species only, another author will subdivide and 

 separate amongst a considerable number. 



And as it is with our knowledge of the genealogy of species, 

 so it is with our knowledge of the character and mode of opera- 

 tion of the agencies which have been employed to bring about 

 their diffusion. Except in those cases where the spreading 

 abroad of species has manifestly been effected by human agency, 

 operating either in a direct or in an indirect manner, we are 

 almost entirely in the dark as to how they have reached the 

 places where we now see them. We cannot tell which are true 

 aboriginal species. We cannot tell whether each true aboriginal 

 species was in its original creation represented by a single 

 individual, or a pair of individuals, or by an indefinite number 

 of individuals, or pairs ; neither, even if we choose to assume the 

 latter to be the case, are we in a position to say to what extent 

 the facts which relate to species-dispersion have been brought 

 about at a period coincident with or posterior to the date of 

 their original creation. 



But although our ignorance is so great, and our chance of 

 diminishing it so small, when in treating of the distribution of 

 species we consider the active agencies by which their diffusion 

 has been brought about, yet when we come to speak of the 

 agencies which have operated to restrict their distribution the 

 case is different. Each species, we can say with confidence, is 

 plainly limited in its distribution by certain physical conditions ; 

 and if we seek out in detail what the physical conditions which 

 accompany the distribution of a species are, we are placed in a 

 position in which we can form something of an estimate as to 

 which of those conditions have upon the distribution an essential 

 bearing. 



The Influence of Human Agency in Modifying a Flora. — In 

 viewing the flora of any definite district, as we have it at the 

 present time, it is needful, if we would wish to inquire respect- 



