M Database Inspector (cheetah)
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|OriginOfSpecies - 475 Rows|
|05 - Laws of Variation||05-08 - Parts Developed in an Unusual Manner are Highly Variable||30||
As with birds the individuals of the same species, inhabiting the same country, vary extremely little, I have particularly attended to them; and the rule certainly seems to hold good in this class.
I cannot make out that it applies to plants, and this would have seriously shaken my belief in its truth, had not the great variability in plants made it particularly difficult to compare their relative degrees of variability.
|06 - Difficutiles in Theory||06-04 - Transitions in Habits of Life||30||
Look at the family of squirrels; here we have the finest gradation from animals with their tails only slightly flattened, and from others, as Sir J. Richardson has remarked, with the posterior part of their bodies rather wide and with the skin on their flanks rather full, to the so-called flying squirrels; and flying squirrels have their limbs and even the base of the tail united by a broad expanse of skin, which serves as a parachute and allows them to glide through the air to an astonishing distance from tree to tree.
We cannot doubt that each structure is of use to each kind of squirrel in its own country, by enabling it to escape birds or beasts of prey, to collect food more quickly, or, as there is reason to believe, to lessen the danger from occasional falls.
But it does not follow from this fact that the structure of each squirrel is the best that it is possible to conceive under all possible conditions.
Let the climate and vegetation change, let other competing rodents or new beasts of prey immigrate, or old ones become modified, and all analogy would lead us to believe that some at least of the squirrels would decrease in numbers or become exterminated, unless they also become modified and improved in structure in a corresponding manner.
Therefore, I can see no difficulty, more especially under changing conditions of life, in the continued preservation of individuals with fuller and fuller flank membranes, each modification being, useful, each being propagated, until, by the accumulated effects of this process of natural selection, a perfect so-called flying squirrel was produced.
|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.
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
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
Another distinguished zoologist, the late Professor Claparide,
has argued in the same manner, and has arrived at the same
He shows that there are parasitic mites (Acaridae), belonging
to distinct sub-families and families, which are furnished with
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||06-06 - Species with Habits Widely Diffferent from those of their Allies||30||
He who believes in separate and innumerable acts of creation may say, that in these cases it has pleased the Creator to cause a being of one type to take the place of one belonging to another type; but this seems to me only re-stating the fact in dignified language.
He who believes in the struggle for existence and in the principle of natural selection, will acknowledge that every organic being is constantly endeavouring to increase in numbers; and that if any one being varies ever so little, either in habits or structure, and thus gains an advantage over some other inhabitant of the same country, it will seize on the place of that inhabitant, however different that may be from its own place.
Hence it will cause him no surprise that there should be geese and frigatebirds with webbed feet, living on the dry land and rarely alighting on the water; that there should be long-toed corncrakes, living in meadows instead of in swamps; that there should be woodpeckers where hardly a tree grows; that there should be diving thrushes and diving Hymenoptera, and petrels with the habits of auks.