DARWIN-WALLACE CENTENARY—pDE BEER 339 
So much of the credit for the establishment of the fact of evolution 
has, rightly, been accorded to Darwin that it is only just that Wal- 
lace’s contribution to this problem should be recognized and honored. 
The evidence on which Darwin and Wallace based their demonstra- 
tion that evolution was a fact is not only valid to this day, but has 
been confirmed in all the branches of science concerned as well as 
in many new fields. There was in their day not even an inkling of 
the possibilities of research opened up by comparative physiology 
and biochemistry, or of serology as a quantitative indicator of the 
amount of divergence that has taken place between related forms. 
Why should the chemical substance involved in the mechanism of 
muscular contraction in most invertebrates be arginine, whereas it 
is creatine in vertebrates and echinoderms, which on independent evi- 
dence are regarded as related? Why should serum immunized 
against man give precipitations of 64 percent when mixed with blood 
of a gorilla, but 42 percent with that of an orangutan, 29 percent with 
that of a baboon, and only 10 percent with that of an ox? Why 
should syphilis attack the chimpanzee more seriously than the orang- 
utan, and the latter more seriously than the baboon? Why should 
the human ABO blood-group system also be found in the apes? The 
answer to all these questions is that the organisms concerned have 
undergone evolution from common ancestors, as a result of which 
members of the various lines of descent share not only structural, men- 
tal, and genetical characters, but also physiological and biochemical 
mechanisms and immunological reactions. 
THE MECHANISM OF NATURAL SELECTION 
Although Darwin already knew in 1837 that evolution was an in- 
escapable conclusion to be drawn from the evidence, he did not allow 
himself to proceed any further with his discovery until he had found 
an explanation of the fact of adaptation. In a general way, all plants 
and animals are adapted to their environment, for otherwise they 
could not live. A man drowns in the sea; a fish dies out of water. 
But there are some structures which show a particularly intimate 
relationship between the organism and its conditions of life. Mistletoe 
is a parasite that requires a tree of certain species to live on, a par- 
ticular insect to pollinate its flowers, and a thrush to eat its berries 
and deposit its seeds on branches of the same species of tree. A wood- 
pecker has two of its toes turned backward with which it grips the 
bark of tree; it has stiff tail feathers with which it props itself against 
the tree; it has a very stout beak with which it bores holes in the tree 
trunk; and it has an abnormally long tongue with which it takes the 
grubs at the bottom of the holes. Other plants than mistletoe and 
other birds than woodpeckers do not have all these adaptations, and 
