DARWIN-WALLACE CENTENARY— DE BEER 353 
flies, when the effects of the “eyeless” gene manifest themselves in all 
their force, though in the second generation, because “eyeless” is reces- 
sive. This result is therefore in perfect accordance with the principle 
that Mendelian genes do not become contaminated. What has hap- 
pened during the inbreeding of “eyeless” flies is that the reshufiling 
of the other genes has produced a gene complex in which the harmful 
effects of “eyeless” have been suppressed. 
The gene complex is therefore a dynamic system, as S. C. Harland 
concluded from his researches on cotton. Genes compete, 1e., are 
selected in the gene complex, old genes being dropped and new genes 
incorporated. During the course of evolution the effective member- 
ship of the gene complex must have changed, and it is not legitimate 
to conclude that because a character or a structure like the eye of 
vertebrates was in existence 400 million years ago, it was then con- 
trolled by the same genes as control it now. The evidence is entirely 
opposed to such a static view. It is precisely because the gene com- 
plexes change that characters, structures, and organisms have evolved. 
THE NEW SYSTEMATICS AND THE ORIGIN OF SPECIES 
The researches on industrial melanism in the peppered moth, band- 
ing and color of snails, mimicry in butterflies, local adaptation in 
moths, and sickle cell in man, which have here been briefly described, 
are examples of new techniques of experimental study of evolution in 
the field. They have grown out of what Julian Huxley has aptly 
called “The New Systematics,” to which he has himself contributed 
somuch. Systematics, the study of species and of the higher groups 
of classification, began by the recognition of differences between 
species, defined from type specimens preserved in museums. But with 
the realization that species now or in the past are or were populations 
of live plants and animals in nature, living under varying conditions 
of equilibrium with each other and with the inorganic factors of the 
environments—themselves showing geographical variation in space, 
and undergoing variation in time, subject to mutation and recombi- 
nation of their genes, constantly under the influence of selection— 
species can no longer be considered as static milestones of evolution, 
for they are themselves the dynamic systems by which the roads of 
evolution are trodden. As genes mutate and are reshufiled, and geo- 
graphical races invade new ecological niches, advance and retreat, 
it is already possible on a map to mark out lines of gene flow, as R. C. 
Stebbins has suggested from his researches on Californian newts. It 
may become possible to plot the areas of gene complex alteration, as 
can to a certain extent already be done for the origin of cultivated 
plants such as wheat; but such maps will be continually changing, like 
the species themselves. 
