ROCKY MOUNTAIN PLANTS 
115 
North America we have Regulus cuvieri, only one specimen 
of which is known, Regulus satrapa and Regulus 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 Regulus invaded North America 
from Asia by way of the Bering Strait land connection. 
Among the invertebrates of the Rocky 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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