adio-activity.

An important consequence of the advance in our knowledge of heredity
associated with the work of Mendel and his successors is a tendency to
doubt whether natural selection is of such fundamental importance in
shaping the course of evolution as was supposed in the years of the
first enthusiasm which followed the publication of the _Origin_.

Darwin based his theory of Natural Selection on the belief which he
derived from breeders of plants and animals, that the kind of variation
used by them to produce new breeds was the small and apparently
unimportant differences which distinguish a 'fine' from a 'poor'
specimen. He supposed that the skilled breeder picked out as parents of
his stock those individuals which were slightly superior in one feature
or another, and that by the accumulative effect of these successive
selections not only was the breed steadily improved, but also, by
divergent selection, new breeds were produced. Experience shows,
however, that although this method is used to keep breeds up to the
required standard, it is rarely, if ever, the means by which new breeds
arise. New breeds commonly come into existence either by a 'sport' or
mutation, or by crossing two already distinct races, and by selecting
from among the heterogeneous descendants of the cross those individuals
which show the required combination of characters. And it is further
found that most of the distinguishing features of various breeds of
domestic animals and plants are inherited according to Mendel's Law,
suggesting that each of these characters is a unit, like one piece of a
mosaic, independent of the rest. Now it is easy to see how the selection
of small, continuously varying characters could take place in Nature by
the destruction of all those individuals which failed to reach a certain
standard, but it is much more difficult to understand how natural
selection could act on comparatively large, sporadic, unco-ordinated
'sports'. There is thus a distinct tendency at present to regard natural
selection as less omnipotent in directing the course of evolution than
was formerly supposed, but it must be admitted that no very satisfactory
alternative hypothesis has been suggested. Some have supposed that there
is a kind of organic momentum which causes evolution to continue in
those directions in which it has already proceeded, while others have
postulated, like Bergson, an _elan vital_ as a kind of directive agency.
Others