8 Darwin, and after Darwin. 
facts, has been well-nigh universally superseded. But 
this great gain has been attended by some measure of 
loss. For while not a few naturalists have since erred 
on the side of insufficiently distinguishing between fully 
verified principles of evolution and merely specula- 
tive deductions therefrom, a still larger number have 
formed for themselves a Darwinian creed, and regard 
any further theorizing on the subject of evolution as 
ipso facto unorthodox. 
Having occupied the best years of my life in 
closely studying the literature of Darwinism, I shall 
endeavour throughout the following pages to avoid 
both these extremes. No one in this generation is 
able to imitate Darwin, either as an observer or a 
generalizer. But this does not hinder that we should 
all so far endeavour to follow his method, as always to 
draw a clear distinction, not merely between observa- 
tion and deduction, but also between degrees of 
verification. At all events, my own aim will every- 
where be to avoid dogmatism on the one hand, and 
undue timidity as regards general reasoning on the 
other. For everything that is said justification will 
be given; and, as far as prolonged deliberation has 
enabled me to do so, the exact value of such justifica- 
tion will be rendered by a statement of at least the 
main grounds on which it rests. The somewhat 
extensive range of the present treatise, however, will 
not admit of my rendering more than a small percen- 
tage of the facts which in each case go to corroborate 
the conclusion. But although a great deal must thus 
be necessarily lost on the one side, I am disposed to 
think that more will be gained on the other, by 
presenting, in a terser form than would otherwise be 
