324 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1926 
grown with remarkable rapidity. It is genetics which must un- 
cover the machinery of evolution, which of course is fundamentally 
a matter of inheritance. The facts thus far uncovered indicate 
complexities which were not realized before, but which should have 
been anticipated, for inheritance, with its resulting evolution, rep- 
resents the most complex biological situation imaginable. 
The present status of evolution as a body of doctrine may be said 
to be in a state of flux, out of which the truth will emerge eventually. 
Any meeting of biologists at which evolution is discussed discloses 
considerable diversity of opinion, not as to the fact of evolution, but 
as to some attempt to explain the process. 
It is evident, of course, that whatever produces variation furnishes 
a basis for evolution. But what produces variation? Environ- 
ment is one factor; sex is another factor, especially when strains 
are crossed; and other factors might be cited. Any factor claimed 
to induce variation must stand the test of genetics. Variations, 
however, produced, are of two general kinds, as indicated by be- 
havior, namely, the so-called continuous variation of Darwin’s ex- 
planation, and the so-called discontinuous variation of DeVries’s ex- 
planation. The differences of opinion have to do with the method 
of variation production; that is, variation that may result in a new 
species. 
After such variation is secured, there is no question as to the func- 
tion of selection. It is merely a statement of fact to say that some 
variations persist and some are eliminated. It is a very different 
matter to claim that only the “ fit” persist. In some way the selec- 
tion is made, and the selection factors may be quite variable. In 
general, it may be said that there is no serious difference of opinion 
that evolution is based on variation and subsequent selection. It 
is only a matter of detail to determine the exact factors. 
There is a much more serious problem of evolution, however, 
which is still baffling. ‘The variations observed, which result in new 
species, as tested by genetics, and for which the cytological ma- 
ehinery has been observed, produce species either laterally or retro- 
gressively; that is, species of the same rank or of declining rank. 
There is as yet no adequate explanation of progressive evolution, the 
advance from one great group to another of higher rank. Progres- 
sive evolution is a very evident fact, as shown by many an im- 
pressive series disclosed by the geological records. The theory of 
“ orthogenesis” is often cited as an attempt to explain progressive 
evolution. Orthogenesis is not an explanation, however, but a name 
for progressive evolution. The fact remains to be explained. The 
multiplication of species is within the reach of experimental study 
as to causes and methods, and the results are leading to conclusions 
