Biology descriptive essay natural nature selection theoretical tropical

The Australian kangaroos are of the same tints, and the original colour of the wild horse is supposed to have been a sandy or clay-colour. Again, if we consider that we have only fragments of this vast system, the stem and main branches being represented by extinct species of which we have no knowledge, while a vast mass of limbs and boughs and minute twigs and scattered leaves is what we have to place in order, so as to determine the true position which each originally occupied with regard to the others, the whole difficulty of the true Natural System of classification becomes apparent to us.

Then I thought of the enormously rapid multiplication of animals, causing these checks to be much more effective in them than in the case of man; and while pondering vaguely on this fact there suddenly flashed upon me the idea of the survival of the fittest—that the individuals removed by these checks must be on the whole inferior to those that survived.

The two are so much opposed to each other in every circumstance of their existence, that what applies to the one is almost sure not to apply to the other. In no other birds are these peculiar circumstances so strikingly combined. Very few birds produce less than two young ones each year, while many have six, eight, or ten; four will certainly be below the average; and if we suppose that each pair produce young only four times in their life, that will also be below the average, supposing them not to die either by violence or want of food.

Rudimentary Organs Another important series of facts, quite in accordance with, and even necessary deductions from, the law now developed, are those of rudimentary organs.

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High Organisation of very ancient Animals consistent with this Law Much discussion has of late years taken place on the question whether the succession of life upon the globe has [page] 12 been from a lower to a higher degree of organisation.

I would also venture to suggest some reasons against the very nature of the theory of Professor Forbes. Why are the closely allied species of brown-backed Trogons all found in the East, and the green-backed in the West?

We reply that these are generally cases of more or less special adaptation. Species of one genus, or genera of one family occurring in the same geological time, are more closely allied than those separated in time.

Our own island is an example of this, its separation Biology descriptive essay natural nature selection theoretical tropical the continent being geologically very recent, and we have consequently scarcely a species which is peculiar to it; while the Alpine range, one of the most recent mountain elevations, separates faunas and floras which scarcely differ more than may be due to climate and latitude alone.

The action of this principle is exactly like that of the centrifugal governor of the steam-engine, which checks and corrects any irregularities almost before they become evident; and in like manner no unbalanced deficiency in the animal kingdom can ever reach any conspicuous magnitude, because it would make itself felt at the very first step, by [page] 33 rendering existence difficult and extinction almost sure soon to follow.

The great increase of our knowledge within the last twenty years, both of the present and past history of the organic world, has accumulated a body of facts which should afford a sufficient foundation for a comprehensive law embracing and [page] 5 explaining them all, and giving a direction to new researches.

It may, however, happen, that though later in time, the new series of species may never attain to so high a degree of organisation as those preceding it, but in its turn become extinct, and give place to yet another modification from the same root, which may be of higher or lower organisation, more or less numerous in species, and more or less varied in form and structure, than either of those which preceded it [page] 13 Again, each of these groups may not have become totally extinct, but may have left a few species, the modified prototypes of which have existed in each succeeding period, a faint memorial of their former grandeur and luxuriance.

Many causes, no doubt, have operated of which we must ever remain in ignorance, and we may, therefore, expect to find many details very difficult of explanation, and in attempting to give one, must allow ourselves to call into our service geological changes which it is highly probable may have occurred, though we have no direct evidence of their individual operation.

Natural selection and tropical nature; essays on descriptive and theoretical biology

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These are the parallel series or representative groups of naturalists, and they often occur in different countries, or are found fossil in different formations.

But it must be remembered that the writer had no opportunity of correcting the proofs of this paper. Change of attitude essay unhygienic food essay journal? In the two hours that elapsed before my ague fit was over I had thought out almost the whole of the theory, and the same evening I sketched the draft of my paper, and in the two succeeding evenings wrote it out in full, and sent it by the next post to Mr.

Thus every case of apparent retrogression may be in reality a progress, though an interrupted one: What have they to do with the great laws of creation? Tristram, in his account of the ornithology of North Africa in the first volume of the Ibis, says: That of a true one is very different, as may be well seen by the progress of opinion on the subject of Natural Selection.

That these really do exist, and in most cases have no special function in the animal economy, is admitted by the first authorities in comparative anatomy. Neither did the giraffe acquire its long neck by desiring to reach the foliage of the more lofty shrubs, and constantly stretching its neck for the purpose, but because any varieties which occurred among its antitypes with a longer neck than usual at once secured a fresh range of pasture over the same ground as their shorter-necked companions, and on the first scarcity of food were thereby enabled to outlive them.

The Form of a true system of Classification determined by this Law If the law above enunciated be true, it follows that the natural series of affinities will also represent the order in which the several species came into existence, each one having had for its immediate antitype a closely allied species existing at the time of its origin.

All we argue for is, that certain varieties have a tendency to maintain their existence longer than the original species, and this tendency, must itself felt; for though the doctrine of chances or averages can never be trusted on a limited scale, yet, if applied to high numbers, the results come nearer to what theory demands, and, as we approach to an infinity of examples, become strictly accurate.

Conclusion It has now been shown though most briefly and imperfectly, how the law that "Every species has come into existence coincident both in time and space with a pre-existing closely allied species," connects together and renders intelligible a vast number of independent and hitherto unexplained facts.

The numbers that die annually must be immense; and as the individual existence of each animal depends upon itself, those that die must be the weakest—the very young, the aged, and the diseased—while those that prolong their existence can only be the most perfect in health and vigour—those who are best able to obtain food regularly, and avoid their numerous enemies.

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More important changes, such as an increase in the power or dimensions of the limbs or any of the external organs, would more or less affect their mode of procuring food or the range of country which they could in- 1 "And that of their offspring" should have been added.Natural Selection and Tropical Nature: Essays On Descriptive and Theoretical Biology - Scholar's Choice Edition [Alfred Russel Wallace] on *FREE* shipping on qualifying offers.

This work has been selected by scholars as being culturally important, and is part of the knowledge base of civilization as we know it. This work was reproduced from the original artifact5/5(1). Tropical Nature and Other Essays: The Climate and Physical Aspects of the Equatorial Zone; Equatorial Vegetation; Animal Life in the Tropical Forests; Hummingbirds: As Illustrating the Luxuriance of Tropical Nature; The Colors of Animals and Sexual Selection; The Colors of Plants and the Origin of the Color-Sense; The Antiquity and Origin of Man; The Antiquity of Man in North America; and The Debt of.

Natural Selection And Tropical Nature; Essays On Descriptive And Theoretical Biology By Alfred Russel Wallace. Natural Selection and Tropical Nature: Essays on Descriptive and Theoretical Biology.

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Biology descriptive essay natural nature selection theoretical tropical
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