[quote author=“Arabian Enlightenment”]The steps that you outlined for us are helpful indeed, so thanks a lot
Nevertheless, I found that, in an argument with a believer, the most difficult wall to overcome is about how scientific the evolution theory is to begin with.
“Variability is not actually caused by man; he only unintentionally exposes organic beings to new conditions of life, and then nature acts on the organization and causes it to vary. But man can and does select the variations given to him by nature, and thus accumulates them in any desired manner.
Man thus adapts animals and plants for his own benefit or pleasure. Man may do this methodically, or he may do it unconsciously by preserving the individuals most useful or pleasing to him without any intention of altering the breed.
It is certain that he can largely influence the character of a breed by selecting, in each successive generation, individual differences so slight as to be inappreciable except by an educated eye. This unconscious process of selection has been the great agency in the formation of the most distinct and useful domestic breeds. That many breeds produced by man have to a large extent the character of natural species, is shown by the inextricable doubts whether many of them are varieties or aboriginally distinct species.
There is no reason why the principles which have acted so efficiently under domestication should not have acted under nature. In the survival of favored individuals and races, during the constantly recurrent struggle for existence, we see a powerful and ever acting form of Selection.
The struggle for existence inevitably follows from the high geometrical ratio of increase which is common to all organic beings. This high rate of increase is proved by calculation, by the rapid increase of many animals and plants during a succession of peculiar seasons, and when naturalized in new countries.
More individuals are born than can possibly survive. A grain in the balance may determine which individuals shall live, and which shall die, which variety or species shall increase in number, and which shall decrease, or finally become extinct.
As the individuals of the same species come in all respects into the closest competition with each other, the struggle will generally be most severe between them; it will be almost equally severe between the varieties of the same species, and next in severity between the species of the same genus.
On the other hand the struggle will often be severe between beings remote in the scale of nature. The slightest advantage in certain individuals, at any age or during any season, over those with which they come into competition, or better adaptation in however slight a degree to the surrounding physical conditions, will, in the long run, turn the balance.
With animals having separated sexes, there will be in most cases a struggle between the males for the possession of the females. The most vigorous males, or those which have most successfully struggled with their conditions of life, will generally leave most progeny. But success will often depend on the males having special weapons, or means of defense, or charms; and a slight advantage will lead to victory.
As geology plainly proclaims that each land has undergone great physical changes, we might have expected to find that organic beings have varied under nature, in the same way as they have varied under domestication.
If there has been any variability under nature, it would be an unaccountable fact if natural selection had not come into play. It has often been asserted, but the assertion is incapable of proof, that the amount of variation under nature is a strictly limited quantity.
Man, though acting on external characters alone and often capriciously, can produce within a short period a great result by adding up mere individual differences in his domestic productions; and everyone admits that species present individual differences. But, besides such differences, all naturalists admit that natural varieties exist, which are considered sufficiently distinct to be worthy of record in systematic works.
No one has drawn any clear distinction between individual differences and slight varieties; or between more plainly marked varieties and sub-species and species. On separate continents, and on different parts of the same continent when divided by barriers of any kind, and on outlying islands, what a multitude of forms exist, which some experienced naturalists rank as varieties, others as geographical races or sub-species, and others as distinct, though closely allied species!
If then, animals and plants do vary, let it be ever so slightly or slowly, why should not variations or individual differences, which are in any way beneficial, be preserved and accumulated through natural selection, or the survival of the fittest?
If man can by patience select variations useful to him, why, under changing and complex conditions of life, should not variations useful to nature’s living products often arise, and be preserved or selected? What limit can be put to this power, acting during long ages and rigidly scrutinizing the whole constitution, structure, and habits of each creature, -favoring the good and rejecting the bad?
I can see no limit to this power, in slowly and beautifully adapting each form to the most complex relations of life. The theory of natural selection, even if we look no farther than this, seems to be in the highest degree probable.”
“As each species tends by its geometrical rate of reproduction to increase inordinately in number; and as the modified descendants of each species will be enabled to increase by as much as they become more diversified in habits and structure, so as to be able to seize on many and widely different places in the economy of nature, there will be a constant tendency in natural selection to preserve the most divergent offspring of anyone species.
Hence, during a long continued course of modification, the slight differences characteristic of varieties of the same species, tend to be augmented into the greater differences characteristic of the species of the same genus. New and improved varieties will inevitably supplant and exterminate the older, less improved, and intermediate varieties; and thus species are rendered to a large extent defined and distinct objects.
Dominant species belonging to the larger groups within each class tend to give birth to new and dominant forms; so that each large group tends to become still larger, and at the same time more divergent in character. But as all groups cannot thus go on increasing in size, for the Earth would not hold them, the more dominant groups beat the less dominant.
This tendency in the large groups to go on increasing in size and diverging in character, together with the inevitable contingency of much extinction, explains the arrangement of all the forms of life in groups subordinate to groups, all within a few great classes, which has prevailed throughout all time.”
“We can to a certain extent understand how it is that there is so much beauty throughout nature; for this may be largely attributed to the agency of selection.
That beauty, according to our sense of it, is not universal, must be admitted by everyone who will look at some venomous snakes, at some fishes, and at certain hideous bats with a distorted resemblance to the human face.
Sexual selection has given the most brilliant colors, elegant patterns, and other ornaments to the males, and sometimes to both sexes, of many birds, butterflies, and other animals. With birds it has often rendered the voice of the male musical to the female, as well as to our ears.
Flowers and fruit have been rendered conspicuous by brilliant colors in contrast with the green foliage, in order that the flowers may be easily seen, visited, and fertilized by insects, and the seeds disseminated by birds.
How it comes that certain colors, sounds, and forms should give pleasure to man and the lower animals, - that is, how the sense of beauty in its simplest form was first acquired, - we do not know any more than how certain odors and flavors were first rendered agreeable.”
“It can hardly be supposed that a false theory would explain, in so satisfactory a manner as does the theory of natural selection, the reason all living things have much in common, in their chemical composition, their cellular structure, their laws of growth, and their liability to injurious influences.
It has recently been objected that this is an unsafe method of arguing; but it is a method used in judging of the common events of life, and has often been used by the greatest natural philosophers. The undulatory theory of light has thus been arrived at; and the belief in the revolution of the Earth on its own axis was until lately supported by hardly any direct evidence.
It is no valid objection that science as yet throws no light on to the far higher problem of the essence or origin of life.”
“When we no longer look at an organic being as a savage looks at a ship, as something wholly beyond his comprehension; when we regard every production of nature as one which has had a long history; when we contemplate every complex structure and instinct as the summing up of many contrivances, each useful to the possessor, in the same way as any great mechanical invention is the summing up of the labor, the experience, the reason, and even the blunders of numerous workmen; when we thus view each organic being, how far more interesting does the study of natural history become!”
“It is interesting to contemplate a tangled bank, clothed with many plants of many kinds, with birds singing on the bushes, with various insects flitting about, and with worms crawling through the damp earth, and to reflect that these elaborately constructed forms, so different from each other, and dependent upon each other in so complex a manner, have all been produced by laws acting around us. “
“There is grandeur in this view of life with its several powers, having been originally breathed by the Creator into a few forms or into one; and that, whilst this Earth has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.”
“False facts are highly injurious to the progress of science, for they often endure long; but false views, if supported by some evidence, do little harm, for everyone takes a salutary pleasure in proving their falseness; and when this is done, one path towards error is closed and the road to truth is often at the same time opened.”
“I see no good reason why the views given in this volume should shock the religious feelings of anyone. It is so easy to hide our ignorance under such expressions as the “plan of creation,” “unity of design,” etc., and to think that we give an explanation when we only restate a fact.”
Charles Darwin, naturalist, excerpts from Origin of the Species