48 



Turning now to comparisons with the North American flora 

 is is rather surprising to find such a long series of species (435!) 

 in common. The number common to Europe (454) is only a 

 little greater. When we list these species, and examine their 

 distribution more in detail, it becomes evident that we are dealing 

 with the circumpolar flora, certain members of which have 

 presumably died out in Europe or America. Some of the 

 species (as Chenopodium album, C. glaucum, A triplex pa tula, 

 Rumex acetosella, etc.) are obviously under suspicion of having 

 been introduced into Yezo by man, though now thorougly 

 established. In some cases it may be that more critical studies 

 would show hitherto unobserved diff^erences. For example, 

 of the 29 species of Polygonum in Yezo, 14 are also reported 

 from North America; but recent investigations show that there 

 are more closely related species of this group than were formerly 

 recognized. Twenty-three of the ferns are also North American 

 and of these sixteen are also European. There is a separate 

 enumeration of the species common to Yezo and Alaska (with 

 the Aleutian Islands), but only 122 (7.49% of the Yezo flora) are 

 listed. The flora of the interior of Alaska is, I suppose, still very 

 imperfectly known, but one would have expected the list to 

 be longer. 



The general outcome seems to be that species of plants are 

 little or slowly affected by mere time or space, but respond rather 

 rapidly to new conditions of life. Or to put it another way, 

 there is little evidence for obligatory evolution, regardless of 

 circumstances.* The age and area postulate does not hold as a 



* This statement has to do with the process of evolution, and not with the 

 causes of mutation. Any plant population, closely studied, exhibits diversity 

 in hereditary qualities, the material for evolution thus being everywhere 

 present. Hybridism produces extraordinary diversity under uniform con- 

 ditions. The breaking down of polyploids is doubtless an important cause 

 of diversity in some genera. Yet the establishment of permanent species 

 (permanent in the sense of lasting for long periods) appears usually to depend 

 on adaptation or response to new conditions. Whether, as Harrison believes, 

 there is a definite chemical response in the germ plasm to diverse substances 

 in the soil, or whether changes occur in a miscellaneous manner and are merely 

 of survival value in the presence of new conditions, the objective results are 

 about the same. 



In my garden is a briar rose {Rosa rubiginosa) which for se\"eral yearsh as 

 been full of the galls of Rhodiles rosae. So close that the twigs touch, is a 

 Rosa rubrijolia, but it has remained wholly unaffected by the galls until the 



