d to the free ends of the caudicles--all these gradations being
of manifest benefit to the plants in question. With respect to climbing
plants, I need not repeat what has been so lately said.

It has often been asked, if natural selection be so potent, why has not
this or that structure been gained by certain species, to which it would
apparently have been advantageous? But it is unreasonable to expect a
precise answer to such questions, considering our ignorance of the past
history of each species, and of the conditions which at the present day
determine its numbers and range. In most cases only general reasons,
but in some few cases special reasons, can be assigned. Thus to adapt
a species to new habits of life, many co-ordinated modifications are
almost indispensable, and it may often have happened that the requisite
parts did not vary in the right manner or to the right degree. Many
species must have been prevented from increasing in numbers through
destructive agencies, which stood in no relation to certain structures,
which we imagine would have been gained through natural selection
from appearing to us advantageous to the species. In this case, as the
struggle for life did not depend on such structures, they could not
have been acquired through natural selection. In many cases complex and
long-enduring conditions, often of a peculiar nature, are necessary for
the development of a structure; and the requisite conditions may seldom
have concurred. The belief that any given structure, which we think,
often erroneously, would have been beneficial to a species, would
have been gained under all circumstances through natural selection, is
opposed to what we can understand of its manner of action. Mr. Mivart
does not deny that natural selection has effected something; but he
considers it as "demonstrably insufficient" to account for the phenomena
which I explain by its agency. His chief arguments have now been
considered, and the others will hereafter be considered. They seem to me
to partake little of the character of demonstration, and to have little
weight in comparison with those in favour of the power of natural
selection, aided by the other agencies often specified. I am bound to
add, that some of the facts and arguments here used by me, have been
advanced for the same purpose in an able article lately published in the
"Medico-Chirurgical Review."

At the present day almost all naturalists admit evolution under som