hmere, but not
northward. Like the _Apollo_, it does not occur in Scandinavia or
Northern Siberia. Both plant and insect evidently migrated from Central
Asia, directly westward along the southern border of the sea, which
extended from that region as far as the European Alps in early Tertiary
times. At that time the Caucasus was possibly still connected with the
Balkan Mountains, across what is now the Black Sea, and that may have
been the highway on which they travelled west.

Some of the Clouded-Yellows--butterflies appertaining to the genus
_Colias_--formed part of the Oriental migration. The genus is
undoubtedly of Asiatic origin, and while many of the species have turned
northward, ranging across Siberia and North America, others have taken a
southern and westward turn and thus reached Europe. We have two
Clouded-Yellows in Western Europe, and both of them must have come with
this migration.

A very good example of an Oriental migrant is _Danais chrysippus_, a
magnificent butterfly found in Greece and Southern Italy. In Asia it is
known from Syria, Persia, and from the whole of the southern portion of
the Continent. The genus _Danais_ (in its wide sense) is a large one,
and principally occurs in the warmer regions of Asia. Three species are
found in North America and only one in Europe.

Among the beetles belonging to this migration, there is one of very
considerable interest from a distributional point of view, for all the
species of the genus--even the whole family to which the genus
belongs--are what is known by zoologists as "Commensalists." These are
animals habitually associating and living in close connection with
others with which they are not tied by any family relations or kinship.
Such a state of close and permanent friendship is called "commensalism."
Now it appears as if the members of this family of beetles
(_Clavigeridae_) had of their own free will formed such a close
connection with colonies of ants--sometimes with one species, sometimes
another. They are the permanent guests of the ants, and in return they
secrete a fluid which is apparently highly prized by them. All of the
_Clavigers_ are provided with peculiar club-shaped antennae, with which
they ungraciously beat their hosts, when they are in want of food.
According to some authorities, they even occasionally gnaw at the pupae
and larvae of the ant with which they live.

Such beetles naturally can only have extremely limited means of
dist