84 THE ENTOMOLOGIST. 
that the series of affinities in nature is a concatenation or 
continuous series; and that though an hiatus is here and 
there observable, this has been caused either by the annihi- 
lation of some original group or species...... or that the 
objects required to fill it up are still in existence, but have 
not yet been discovered.” Modern naturalists find in this 
more or less gradual blending their strongest argument in 
favour of community of descent; and speculation as to the 
origin, or outcome rather, in the near present or remote past, 
of existing forms, is naturally and very generally indulged, 
even by those who a few years back were more inclined to 
ridicule than accept Darwinian doctrine. Shall we then say 
that the old divisions must be discarded because not abso- 
lute? As well might we argue for the abolition of the four 
seasons because they differ with the latitude, or because they 
gradually blend into each other. Entomologists will always 
speak of moths and butterflies, howsoever arbitrary the groups 
may come to be looked upon, or however numerous the 
intermediate gradations. These thoughts naturally present - 
themselves in considering so osculant a species as the Yucca 
borer. 
The entomological reader is aware that the queenly 
Yuecas cradle and nourish a very curious and anomalous 
Lepidopteron—the Pronuba yuccasella. The genus is further 
interesting, from the entomological side, as giving us the 
insect under consideration. In the home of the Yuccas, and 
more particularly in the home of the caulescent species, like 
Y. aloifolia and Y. gloriosa, persons who have occasion to dig 
up the roots, or subterranean trunks, often notice that these 
are bored and hollowed out along the axis, the burrow 
cylindrical, and lined at its upper end with silk, which is 
generally intermixed with a white, glistening, soapy powder. 
These tunnellings are made by our Yucca borer, which 
dwells therein; and their presence may generally be detected 
by masses of excrement observable amongst the leaves, and 
by certain chimney-like projections made by the twisting and 
webbing together of the more tender heart-leaves, or even of 
the flower-stalk, after they have been partly devoured, into a 
sort of funnel, from which the excrement is expelled. ‘The 
tunnellings weaken the trunk and induce rot, so that the 
plant is not unfrequently prostrated thereby; and as the 
