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OriginOfSpecies - 475 Rows
Column Type #Values Column Stats
id int(11) 475 Column Stats
subject varchar(80) 14 Column Stats
title varchar(250) 139 Column Stats
ordinal int(11) 30 Column Stats
description text 474 Column Stats

475 rows, page 46 of 119 (4/p)
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05 - Laws of Variation 05-08 - Parts Developed in an Unusual Manner are Highly Variable 40 When we see any part or organ developed in a remarkable degree or manner in a species, the fair presumption is that it is of high importance to that species: nevertheless it is in this case eminently liable to variation. Why should this be so?

On the view that each species has been independently created, with all its parts as we now see them, I can see no explanation.

But on the view that groups of species are descended from some other species, and have been modified through natural selection, I think we can obtain some light.

First let me make some preliminary remarks.

If, in our domestic animals, any part or the whole animal be neglected, and no selection be applied, that part (for instance, the comb in the Dorking fowl) or the whole breed will cease to have a uniform character: and the breed may be said to be degenerating.

In rudimentary organs, and in those which have been but little specialised for any particular purpose, and perhaps in polymorphic groups, we see a nearly parallel case; for in such cases natural selection either has not or cannot have come into full play, and thus the organisation is left in a fluctuating condition.

But what here more particularly concerns us is, that those points in our domestic animals, which at the present time are undergoing rapid change by continued selection, are also eminently liable to variation.

Look at the individuals of the same breed of the pigeon, and see what a prodigious amount of difference there is in the beaks of tumblers, in the beaks and wattle of carriers, in the carriage and tail of fantails, &c., these being the points now mainly attended to by English fanciers.

Tumbler Pigeon
Tumbler Pigeon

English Carrier Pigeon
English Carrier Pigeon

Fantail Pigeon
Fantail Pigeon


Even in the same sub-breed, as in that of the short-faced tumbler, it is notoriously difficult to breed nearly perfect birds, many departing widely from the standard.

Short Faced Tumbler Pigeon
Short Faced Tumbler Pigeon


There may truly be said to be a constant struggle going on between, on the one hand, the tendency to reversion to a less perfect state, as well as an innate tendency to new variations, and, on the other hand, the power of steady selection to keep the breed true. In the long run selection gains the day, and we do not expect to fail so completely as to breed a bird as coarse as a common tumbler pigeon from a good short-faced strain.

But as long as selection is rapidly going on, much variability in the parts undergoing modification may always be expected.
06 - Difficutiles in Theory 06-04 - Transitions in Habits of Life 40 Now look at the Galeopithecus or so-called flying lemur, which formerly was ranked amongst bats, but is now believed to belong to the Insectivora.
lemur
lemur

Flying Lemur
Flying Lemur


An extremely wide flank membrane stretches from the corners of the jaw to the tail, and includes the limbs with the elongated fingers. This flank-membrane is furnished with an extensor muscle.

Although no graduated links of structure, fitted for gliding through the air, now connect the Galeopithecus with the other Insectivora, yet there is no difficulty in supposing that such links formerly existed, and that each was developed in the same manner as with the less perfectly gliding squirrels; each grade of structure having been useful to its possessor.

Nor can I see any insuperable difficulty in further believing that the membrane connected fingers and fore-arm of the Galeopithecus might have been greatly lengthened by natural selection; and this, as far as the organs of flight are concerned, would have converted the animal into a bat.
bat
bat


In certain bats in which the wing-membrane extends from the top of the shoulder to the tail and includes the hind-legs, we perhaps see traces of an apparatus originally fitted for gliding through the air rather than for flight.
06 - Difficutiles in Theory 06-08 - Means of Transition 40 According to this view it may be inferred that all vertebrate animals with true lungs are descended by ordinary generation from an ancient and unknown prototype, which was furnished with a floating apparatus or swimbladder.

We can thus, as I infer from Owen's interesting description of these parts, understand the strange fact that every particle of food and drink & which we swallow has to pass over the orifice of the trachea, with some risk of falling into the lungs, notwithstanding the beautiful contrivance by which the glottis is closed. In the higher Vertebrate the branchiae have wholly disappeared- but in the embryo the slits on the sides of the neck and the loop-like course of the arteries still mark their former position.

But it is conceivable that the now utterly lost branchiae might have been gradually worked in by natural selection for some distinct purpose: for instance, Landois has shown that the wings of insects are developed from the tracheae; it is therefore highly probable that in this great class organs which once served for respiration have been actually converted into organs for flight.
gills
gills

Swim Bladder
Swim Bladder

lungs
lungs

wings
wings
06 - Difficutiles in Theory 06-11 - Organs of Small Importance 40 In the second place, we may easily err in attributing importance to characters, and in believing that they have been developed through natural selection.

We must by no means overlook the effects of the definite action of changed conditions of life,- of so-called spontaneous variations, which seem to depend in a quite subordinate degree on the nature of the conditions,- of the tendency to reversion to long-lost characters,- of the complex laws of growth, such as of correlation, compensation, of the pressure of one part on another, &c.,- and finally of sexual selection, by which characters of use to one sex are often gained and then transmitted more or less perfectly to the other sex, though of no use to this sex.

But structures thus indirectly gained, although at first of no advantage to a species, may subsequently have been taken advantage of by its modified descendants, under new conditions of life and newly acquired habits.