DARWIN-WALLACE CENTENARY—bpdE BEER 337 
Knowledge of the fossil record in Darwin’s time was so imperfect 
that nothing was then available in the way of series illustrating the 
course of evolution. Nevertheless, he noticed that in Tertiary strata 
the lower the horizon the fewer fossils there were belonging to species 
alive today. Paleontology therefore showed that new species had 
appeared and old species become extinct, not all at the same time, but 
in succession and gradually. Why should this be so unless new species 
have come into existence from time to time by descent with modifica- 
tion from other species ? 
Plants and animals are classified according to their resemblance, 
and they are placed in one or another of a not very large number of 
groups, such as ferns, conifers, mollusks, or mammals. But within 
each of these groups there is subdivision into other smaller groups, 
mammals being so subdivided into rodents, carnivores, ungulates, and 
primates for example. Within these again there is further subdivision, 
and the important point to notice is that classification always places 
species in groups that are contained within other larger groups. This 
is such a commonplace that its significance is often overlooked. Why 
do organisms have to be classified like this? Why are they not strewn 
in single file up the ladder of the plant and animal kingdoms, or 
fortuitously like pebbles on a beach, or arbitrarily like the stars in 
imaginary constellations? The reason is that the arrangement of 
groups within groups is a natural classification reflecting the course 
of evolution. It is the result of descent from common ancestors and 
an indication of affinity; the differences between the groups are due 
to modification and divergence during such descent. 
Darwin also investigated the problem of interspecific sterility and 
saw that it was by no means absolute, because numerous examples can 
be found of different species that produce hybrids, and in some cases 
these hybrids are themselves fertile. From the point of view of breed- 
ing, therefore, such species behave like varieties. Why, then, can 
species not have originated as varieties, by descent and modification 
from other species ? 
From the evidence provided by all these sources Darwin built up 
an irrefutable argument that species have changed and originated 
from other species and that evolution has occurred. That he should 
have been able to do so from such few data is a mark of genius, for 
at the time when he worked out his conclusions, none of the cases had 
been discovered which would now be used as the most striking examples 
with which to illustrate the fact and the course of evolution. Chief 
among these are the beautiful series of fossils which reveal the evolu- 
tion of the ammonites or of the horses, step by step, and those which 
represent the precursors of the various classes and groups of verte- 
brates such as Archaeopteryx or Pithecanthropus. 
