404 BIOLOGY AND ITS MAKERS 
may be admitted, there is still abundant evidence of evolu- 
tion without mutation. 
Reconciliation of Different Theories.—All this is leading 
to a clearer appreciation of the points involved in the dis- 
cussion of the theories of evolution; the tendency is not for 
the breach between the different theories to be widened, but 
for evolutionists to realize more fully the great complexity 
of the process they are trying to explain, and to see that no 
single factor can carry the burden of an explanation. Muta- 
tion is not a substitute for natural selection, but a codperating 
factor; and neither mutation nor natural selection is a sub- 
stitute for the doctrine of the continuity of the germ-plasm. 
Thus we may look forward to a reconciliation between 
apparently conflicting views, when naturalists by sifting 
shall have determined the truth embodied in the various 
theories. One conviction that is looming into prominence is 
that this will be promoted by less argument and more ex- 
perimental observation. 
That the solution of the underlying question in evolution 
will still require a long time is evident; as Whitman said 
in his address before the Congress of Arts and Science in 
St. Louis in 1904: ‘The problem of problems in biology 
to-day, the problem which promises to sweep through the 
present century as it has the past one, with cumulative inter- 
est and correspondingly important results, is the one which 
became the life-work of Charles Darwin, and which can not 
be better or more simply expressed than in the title of his 
epoch-making book, The Origin of Species.” 
Summary.—The number of points involved in the four 
theories considered above is likely to be rather confusing, 
and we may now bring them into close juxtaposition. The 
salient features of these theories are as follows: 
