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06 - Difficutiles in Theory 06-09 - Cases of Difficulty 30 Fritz Muller, in order to test the conclusions arrived at in this volume, has followed out with much care a nearly similar line of argument.

Several families of crustaceans include a few species, possessing an air-breathing apparatus and fitted to live out of the water.

crustacean
crustacean


In two of these families, which were more especially examined by Muller and which are nearly related to each other, the species agree most closely in all important characters; namely, in their sense organs, circulating system, in the position of the tufts of hair within their complex stomachs, and lastly in the whole structure of the water-breathing branchiae, even to the microscopical hooks by which they are cleansed.

Hence it might have been expected that in the few species
belonging to both families which live on the land, the equally
important air-breathing apparatus would have been the same;
for why should this one apparatus, given for the same
purpose, have been made to differ, whilst all the other
important organs were closely similar or rather identical?

Fritz Muller argues that this close similarity in so many
points of structure must, in accordance with the views
advanced by me, be accounted for by inheritance from a
common progenitor.

But as the vast majority of the species in the above two
families, as well as most other crustaceans, are aquatic in
their habits, it is improbable in the highest degree, that their
common progenitor should have been adapted for breathing
air was thus led carefully to examine the apparatus in the
air-breathing species; and he found it to differ in each in
several important points, as in the position of the orifices, in
the manner in which they are opened and closed, and in
some accessory details.

Now such differences are intelligible, and might even have
been expected, on the supposition that species belonging to
distinct families had slowly become adapted to live more and
more out of water, and to breathe the air.

For these species, from belonging to distinct families, would
have differed to a certain extent, and in accordance with the
principle that the nature of each variation depends on two
factors, viz., the nature of the organism and that of the
surrounding conditions, their variability assuredly would not
have been exactly the same.

Consequently natural selection would have had different
materials or variations to work on, in order to arrive at the
same functional result; and the structures thus acquired
would almost necessarily have differed.

On the hypothesis of separate acts of creation the whole
case remains unintelligible.

This line of argument seems to have had great weight in
leading Fritz Muller to accept the views maintained by me in
this volume.

Another distinguished zoologist, the late Professor Claparide,
has argued in the same manner, and has arrived at the same
result.

He shows that there are parasitic mites (Acaridae), belonging
to distinct sub-families and families, which are furnished with
hair-claspers.

mite
mite


These organs must have been independently developed, as
they could not have been inherited from a common progenitor;
and in the several groups they are formed by the modification
of the fore-legs,- of the hind-legs,- of the maxillae or lips,- and
of appendages on the under side of the hind part of the body.
06 - Difficutiles in Theory 06-11 - Organs of Small Importance 30 Organs now of trifling importance have probably in some cases been of high importance to an early progenitor, and, after having been slowly perfected at a former period, have been transmitted to existing species in nearly the same state, although now of very slight use; but any actually injurious deviations in their structure would of course have been checked by natural selection.

Seeing how important an organ of locomotion the tail is in most aquatic animals, its general presence and use for many purposes in so many land animals, which in their lungs or modified swimbladders betray their aquatic origin, may perhaps be thus accounted for.
cheetah
cheetah


A well-developed tail having been formed in an aquatic animal, it might subsequently come to be worked in for all sorts of purposes,- as a fly-flapper, an organ of prehension, or as an aid in turning, as in the case of the dog, though the aid in this latter respect must be slight, for the hare, with hardly any tail, can double still more quickly.
06 - Difficutiles in Theory 06-12 - Organs not in all Cases Absolutely Perfect 30 So we may believe that the progenitor of the seal did not possess a flipper, but a foot with five toes fitted for walking or grasping; but we may further venture to believe that the several bones in the limbs of the monkey, horse, and bat, were originally developed, on the principle of utility, probably through the reduction of more numerous bones in the fin of some ancient fish-like progenitor of the whole class.
seal
seal

monkey
monkey

horse
horse

bat
bat


It is scarcely possible to decide how much allowance ought to be made for such causes of change, as the definite action of external conditions, so-called spontaneous variations, and the complex laws of growth; but with these important exceptions, we may conclude that the structure of every living creature either now is, or was formerly, of some direct or indirect use to its possessor.
06 - Difficutiles in Theory 06-13 - Summary: The Law of Unity of Type and of the Conditions of Existence Embraced by the Theory of Natural Selection 30 We have seen that a species under new conditions of life may change its habits; or it may have diversified habits, with some very unlike those of its nearest congeners. Hence we can understand, bearing in mind that each organic being is trying to live wherever it can live, how it has arisen that there are upland geese with webbed feet, ground woodpeckers, diving thrushes, and petrels with the habits of auks.
goose
goose

woodpecker
woodpecker

thrush
thrush

petrel
petrel

auk
auk