first appears monstrous and to
<the> end appears difficulty. But think of gradation, even now manifest,
(Tibia and Fibula). Everyone will allow if every fossil preserved,
gradation infinitely more perfect; for possibility of selection a
perfect <?> gradation is required. Different groups of structure, slight
gradation in each group,--every analogy renders it probable that
intermediate forms have existed. Be it remembered what strange
metamorphoses; part of eye, not directly connected with vision, might
come to be [thus used] gradually worked in for this end,--swimming
bladder by gradation of structure is admitted to belong to the ear
system,--rattlesnake. [Woodpecker best adapted to climb.] In some cases
gradation not possible,--as vertebrae,--actually vary in domestic
animals,--less difficult if growth followed. Looking to whole animals, a
bat formed not for flight{85}. Suppose we had flying fish{86} and not
one of our now called flying fish preserved, who would have guessed
intermediate habits. Woodpeckers and tree-frogs both live in countries
where no trees{87}.

{84} In the _Origin_ a chapter is given up to "difficulties on
theory": the discussion in the present essay seems slight even when
it is remembered how small a space is here available. For _Tibia_
&c. see p. 48.

{85} This may be interpreted "The general structure of a bat is the
same as that of non-flying mammals."

{86} That is truly winged fish.

{87} The terrestrial woodpecker of S. America formed the subject of
a paper by Darwin, _Proc. Zool. Soc._, 1870. See _Life and
Letters_, vol. iii. p. 153.

The gradations by which each individual organ has arrived at its present
state, and each individual animal with its aggregate of organs has
arrived, probably never could be known, and all present great
difficulties. I merely wish to show that the proposition is not so
monstrous as it at first appears, and that if good reason can be
advanced for believing the species have descended from common parents,
the difficulty of imagining intermediate forms of structure not
sufficient to make one at once reject the theory.

Sec. III. <ON VARIATION IN INSTINCTS AND OTHER MENTAL ATTRIBUTES.>

The mental powers of different animals in wild and tame state [present
still greater difficulties] require a separate section. Be it remembered
I have nothing to do with origin of memory, attention, and the different
facult