Evidences of Theory of Natural Selection. 287 
be proved to be of exclusive use to any species other 
than the one presenting it. He even went so far as to 
say that if any one such instance could be shown he 
would surrender his whole theory on the strength of 
it—so assured had he become, by his own prolonged 
researches, that natural selection was the true agent 
in the production of adaptive structures, and, as such, 
could never have permitted such a structure to occur 
in one species for the benefit of another. Now, as this 
invitation has been before the world for so many years, 
and has not yet been answered by any naturalist, we 
may by this time be pretty confident that it never will 
be answered. Howtremendous, then, is the significance 
of this fact in its testimony to Darwin's theory! The 
number of animal and vegetable species, both living 
and extinct, is to be reckoned by millions, and every 
one of these species presents on an average hundreds of 
adaptive structures,—at least one of which in many, 
possibly in most, if not actually in all cases, is peculiar 
to the species that presents it. In other words, there 
are millions of adaptive structures (not to speak of in- 
stincts) which are peculiar to the species presenting 
them, and also many more which are the common 
property of allied species: yet, notwithstanding this 
inconceivable profusion of adaptive structures in 
organic nature, there is no single instance that has 
been pointed out of the occurrence of such a structure 
save for the benefit of the species that presents it. 
Therefore, I say that this immensely large and general 
fact speaks with literally immeasurable force in favour 
of natural selection, as at all events one of the main 
causes of organic evolution. For the fact is precisely 
what we should expect if this theory is true, while 
