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|02 - Variations Under Nature||02-04 - Wide-ranging, much diffused, and common Species vary most||20||
Alphonse de Candolle and others have shown that plants which have very wide ranges generally present varieties; and this might have been expected, as they are exposed to diverse physical conditions, and as they come into competition (which, as we shall hereafter see, is an equally or more important circumstance) with different sets of organic beings.
But my tables further show that, in any limited country, the species which are the most common, that is abound most in individuals, and the species which are most widely diffused within their own country (and this is a different consideration from wide range, and to a certain extent from commonness), oftenest give rise to varieties sufficiently well marked to have been recorded in botanical works.
Hence it is the most flourishing, or, as they may be called, the dominant species,- those which range widely, are the most diffused in their own country, and are the most numerous in individuals,- which oftenest produce well-marked varieties, or, as I consider them, incipient species.
And this, perhaps, might have been anticipated; for as varieties, in order to become in any degree permanent, necessarily have to struggle with the other inhabitants of the country, the species which are already dominant will be the most likely to yield offspring, which, though in some slight degree modified, still inherit those advantages that enabled their parents to become dominant over their compatriots.
In these remarks on predominance, it should be understood that reference is made only to the forms which come into competition with each other, and more especially to the members of the same genus or class having nearly similar habits of life.
|14 - Recapitulation and Conclusion||14-03 - Causes of the general belief in the immutability of species||130||
Although I am fully convinced of the truth of the views given in this volume under the form of an abstract, I by no means expect to convince experienced naturalists whose minds are stocked with a multitude of facts all viewed, during a long course of years, from a point of view directly opposite to mine.
It is so easy to hide our ignorance under such expressions as the `plan of creation,' `unity of design,' &c., and to think that we give an explanation when we only restate a fact.
Any one whose disposition leads him to attach more weight to unexplained difficulties than to the explanation of a certain number of facts will certainly reject my theory.
A few naturalists, endowed with much flexibility of mind, and who have already begun to doubt on the immutability of species, may be influenced by this volume; but I look with confidence to the future, to young and rising naturalists, who will be able to view both sides of the question with impartiality.
Whoever is led to believe that species are mutable will do good service by conscientiously expressing his conviction; for only thus can the load of prejudice by which this subject is overwhelmed be removed.
Several eminent naturalists have of late published their belief that a multitude of reputed species in each genus are not real species; but that other species are real, that is, have been independently created.
This seems to me a strange conclusion to arrive at.
They admit that a multitude of forms, which till lately they themselves thought were special creations, and which are still thus looked at by the majority of naturalists, and which consequently have every external characteristic feature of true species, -- they admit that these have been produced by variation, but they refuse to extend the same view to other and very slightly different forms.
Nevertheless they do not pretend that they can define, or even conjecture, which are the created forms of life, and which are those produced by secondary laws.
They admit variation as a vera causa in one case, they arbitrarily reject it in another, without assigning any distinction in the two cases.
The day will come when this will be given as a curious illustration of the blindness of preconceived opinion.
These authors seem no more startled at a miraculous act of creation than at an ordinary birth.
But do they really believe that at innumerable periods in the earth's history certain elemental atoms have been commanded suddenly to flash into living tissues?
Do they believe that at each supposed act of creation one individual or many were produced?
Were all the infinitely numerous kinds of animals and plants created as eggs or seed, or as full grown?
and in the case of mammals, were they created bearing the false marks of nourishment from the mother's womb?
Although naturalists very properly demand a full explanation of every difficulty from those who believe in the mutability of species, on their own side they ignore the whole subject of the first appearance of species in what they consider reverent silence.
|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||40||
Although the belief that an organ so perfect as the eye could have been formed by natural selection, is enough to stagger any one; yet in the case of any organ, if we know of a long series of gradations in complexity, each good for its possessor, then, under changing conditions of life, there is no logical impossibility in the acquirement of any conceivable degree of perfection through natural selection.
In the cases in which we know of no intermediate or transitional states, we should be extremely cautious in concluding that none can have existed, for the metamorphoses of many organs show what wonderful changes in function are at least possible.
For instance, a swimbladder has apparently been converted into an air-breathing lung.
The same organ having performed simultaneously very different functions, and then having been in part or in whole specialised for one function; and two distinct organs having performed at the same time the same function, the one having been perfected whilst aided by the other, must often have largely facilitated transitions.
|06 - Difficutiles in Theory||06-09 - Cases of Difficulty||10||
Although we must be extremely cautious in concluding that any organ could not have been produced by successive, small, transitional gradations, yet undoubtedly serious cases of difficulty occur.
One of the most serious is that of neuter insects, which are often differently constructed from either the males or fertile females; but this case will be treated of in the next chapter.
The electric organs of fishes offer another case of special difficulty; for it is impossible to conceive by, what steps these wondrous organs have been produced.
But this is not surprising, for we do not even know of what use they are.
In the Gymnotus and torpedo they no doubt serve as powerful means of defence, and perhaps for securing prey; yet in the ray, as observed by Matteucci, an analogous organ in the tail manifests but little electricity, even when the animal is greatly irritated; so little, that it can hardly be of any use for the above purposes.
Moreover, in the ray, besides the organ just referred to, there is, as Dr. R. McDonnell has shown, another organ near the head, not known to be electrical, but which appears to be the real homologue of the electric battery in the torpedo.
It is generally admitted that there exists between these organs and ordinary muscle a close analogy, in intimate structure, in the distribution of the nerves, and in the manner in which they are acted on by various reagents.
It should, also, be especially observed that muscular contraction is accompanied by an electrical discharge; and, as Dr. Radcliffe insists, "in the electrical apparatus of the torpedo during rest, there would seem be a charge in every respect like that which is met with in muscle and nerve during rest, and the discharge of the torpedo, instead of being peculiar, may be only another form of the discharge which depends upon the action of muscle and motor nerve."
Beyond this we cannot at present go in the way of explanation; but as we know so little about the uses of these organs, and as we know nothing about the habits and structure of the progenitors of the existing electric fishes, it would be extremely bold to maintain that no serviceable transitions are possible by which these organs might have been gradually developed.