ROCKY MOUNTAIN PLANTS 115 



North America wo have Eegulus cuvieri, only one specimen 

 of which is known, Eegulus satrapa and Eegulus calendula. 

 None of these have succeeded in penetrating southward 

 beyond Guatemala in Central America, the genus having a 

 wide range on the continent. Hence it is reasonable to 

 suppose that the ancestors of Eegulus invaded North America 

 from Asia by way of the Bering Strait land connection. 



Among the invertebrates of the Eocky Mountains the 

 beetles and butterflies are probably the best-known groups. 

 They may be considered by some as of little importance in the 

 solution of such problems as we have been dealing with., 

 because these insects are generally believed to be very liable 

 to accidental dispersal. One of the most powerful distri- 

 buting agents of insects subject to accidental dispersal is no 

 doubt the wind. Nevertheless many naturalists, who have 

 made a serious study of the geographical distribution of 

 animals and plants, have come to the conclusion that neither 

 wind nor other agencies of accidental dispersal are of such 

 paramount importance as we are often led to believe. The 

 species of a genus, even of butterflies or beetles, as a rule, 

 are clustered round a centre from which we can easily 

 imagine them to have been slowly dispersed in the course 

 of time. Usually we can trace an intimate relationship 

 between the species whose areas of distribution adjoin 

 one another. The conditions of dispersal, in fact, even 

 among winged insects, must be quite similar to those 

 with which we are acquainted among the higher mam- 

 mals. The latter spread gradually on land from their 

 centre of origin. Sometimes we meet with allied groups 

 of species among beetles and butterflies whose ranges are 

 separated by extremely wide areas in which no near relations 

 occur. We might be tempted to attribute such instances to 

 accidental dispersal by wind. We might suppose that <an 

 exceptionally powerful storm had carried these frail insects 

 a few thousand miles away to a spot, where on alighting they 

 found the conditions for their future development favourable. 

 When similar cases of distribution occur among mammals 

 they are explained in a different manner. We then argue 

 that the related, but now widely separated or " discontinuous," 

 groups must long ago have had a perfectly continuous range 



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