INFLUENCE OF ISOLATION ON THE FORMATION OF SPECIES 297 



this must always be so. The species which has been carried to 

 a strange area need not always contain particular variational ten- 

 dencies in its germ-plasm, and need not in every case be impelled to 

 such variations by the influence of new conditions. We know species 

 which have made their way into new regions, and, without var^dng at 

 all, have held their own with, or even proved superior to, the species 

 which were already settled there. Many cases of this kind are 

 known, both among plants and animals ; these have been brouo-ht 

 by man, intentionally or by chance, from one continent to another, 

 and have established themselves and spread over the new area. I 

 need only recall the evening primrose (CEnothera biennis'^), whose 

 fatherland is Virginia, but whose beautiful big yellow blossoms now 

 display themselves beside nearly every river in Germany, having 

 migrated stream-upwards along the gravelly soil ; or the troublesome 

 weed (Erigeron canadense), which is now scarcely less common in our 

 gardens than in those of Canada ; or the sparrow [Passer domestictis), 

 which was introduced into the United States to destroy the cater- 

 pillars, but which preferred instead to plunder the rich stores of corn, 

 and in consequence of these favourable conditions increased to such 

 an extent that it has now become a veritable pest, all imaginable 

 means for its extirpation having been tried— as yet, however, with no 

 great I'esults. 



In all these cases the migration is certainly of recent date, and 

 it is quite possible that, when a longer time has elapsed, some 

 , variations will take place in the new home, but in any case these 

 instances prove that an immigrant species can spread over its new 

 area without immediately varying. 



Similarly, it must be admitted that species which have belonged 

 to two continents ever since Tertiary times need not have diverged 

 since that time, and we know^ for instance, thirty-two species of 

 nocturnal Lepidoptera which are common to North America and to 

 Europe and yet exhibit no differences, while twenty-seven other 

 nocturnal Lepidoptera are, according to Grote, represented in America 

 by ' vicarious ' species, that is, by species which have varied slightly 

 in one or other of the two areas, perhaps in both. 



To sum up : we must undoubtedly admit that isolation has 

 a considerable influence in the evolution of species, though onl}^ in 

 association with selection in its various grades and modes, especially 



^ This was written before the appearance of the researches which De Vries has 

 made on the variations of CEnothera in Europe. Tlius the illustration may not be quite 

 apposite, for it seems to remain undetermined whether the ' mutations ' whicli occur 

 in Holland do not also occasionally appear in America. See end of lecture xxxiii. 



