temporal muscles have left no marks whatever, either by
limiting lines or by the character of the surfaces covered; and the
places of attachment for the masseter muscles are very feebly developed.
At the Museum of Natural History, among skulls of dogs there is one
which, though unnamed, is shown by its small size and by its teeth, to
have belonged to one variety or other of lap-dogs, and which has the
same traits in an equal degree with the skull just described. Here,
then, we have two if not three kinds of dogs which, similarly leading
protected and pampered lives, show that in the course of generations the
parts concerned in clenching the jaws have dwindled. To what cause must
this decrease be ascribed? Certainly not to artificial selection; for
most of the modifications named make no appreciable external signs: the
width across the zygomata could alone be perceived. Neither can natural
selection have had anything to do with it; for even were there any
struggle for existence among such dogs, it cannot be contended that any
advantage in the struggle could be gained by an individual in which a
decrease took place. Economy of nutrition, too, is excluded. Abundantly
fed as such dogs are, the constitutional tendency is to find places
where excess of absorbed nutriment may be conveniently deposited, rather
than to find places where some cutting down of the supplies is
practicable. Nor again can there be alleged a possible correlation
between these diminutions and that shortening of the jaws which has
probably resulted from selection; for in the bull-dog, which has also
relatively short jaws, these structures concerned in closing them are
unusually large. Thus there remains as the only conceivable cause, the
diminution of size which results from diminished use. The dwindling of a
little-exercised part has, by inheritance, been made more and more
marked in successive generations.

* * * * *

Difficulties of another class may next be exemplified--those which
present themselves when we ask how there can be effected by the
selection of favourable variations, such changes of structure as adapt
an organism to some useful action in which many different parts
co-operate. None can fail to see how a simple part may, in course of
generations, be greatly enlarged, if each enlargement furthers, in some
decided way, maintenance of the species. It is easy to understand, too,
how a complex part, as an entire