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fore bo more likely than a part which has not recently varied to vary still farther.
Walsh has called attention ("Proc. Entomolog. Soc," Philadelphia, October, 1863, p. 213) to what he calls the "Law of Equable Variation," which is, "if any given character is very variable in one species of a group, it will tend to be variable in allied species, and if any given character is perfectly constant in one species of a group, it toill tend to be constant in allied species."
This is by no means an absolute law, but simply a general rule. Darwin points out that something of the same kind occurs in domesticated races, and that in the forms which are now undergoing rapid improvement those parts or characters which are most valued vary the most.
We can readily see that circumstances which cause a certain part to throw off gemmules, and thus induce variability, in one species, will be likely to produce the same effect on allied species living under similar circumstances. We can also understand that the divergent modification which has resulted in the formation of several species or races from a parent form, will in itself be a cause of still further modification in the same general direction.
Another well-known law, of which many examples will be given in Chapter IX. is that secondary sexual characters are highly variable. In the chapter on this subject I shall show that the distinctive sexual characters of a species are usually due to recent modification. Their great variability is therefore due to the same cause as that which renders specific characters more variable than generic, and is exactly what our theory would lead us to expect.
Natural Selection cannot produce Race Modification unless the Same Part tends to vary in a Number of Individuals at the Same Time.
This argument, which seems to me to be the most important one which has ever been adduced against the theory of natural selection, was first advanced by a writer in the North British Review in June, 1876.
The author points out that since the chance of survival of any particular individual which is born is very slight indeed, the birth of an individual with any particular slight advantage, and its consequent superiority over its fellows, would not be sufficient to over-balance the chance of its destruction. The objection, which is purely logical, and not experimental, will be stated at length in another place. At present the fact that those who are best qualified to judge, Darwin among them, have acknowledged its great weight, will suffice to show that it is a real and valid objection, and that the foothold of the theory of natural selection would be greatly strengthened if we could show that the causes which produce variation act in such a way as to cause the same part to vary at the same time in great numbers of individuals.
According to our theory of heredity, this will generally be the case. We suppose that an unfavorable change in the environment of a particular cell causes this cell to throw off gemmules. It is plain that a change in the external world, which unfavorably affects any particular cell or group of cells in one individual, will usually affect the corresponding cells of other individuals of the species at the same time. When any particular cell is prolific of gemmules in one individual of a species, the same thing will usually be true of the same cell in other individuals, and the corresponding cell will therefore be a hybrid, and will tend to vary in many descendants.
In each of these descendants this hybrid will be composed of almost identical elements, and they will all tend to vary in the same or nearly the same manner; and as each variation causes other cells to throw off gemmules, the number of individuals which are similarly modified will tend to increase from generation to generation, and natural selection will therefore act, not on a single exceptional individual, but upon a great number, all of 'which are modified in essentially the same way.
If Variation is Purely Fortuitous, the Evohition of a Complicated Organ composed of Many Parts by Natural Selection demands a Period of Time which is almost Infinite.
This obvious objection to the law of natural selection has been so frequently discussed that it is unnecessary to dwell upon it at present, especially as I shall examine it in detail in another place. At present I will only call attention to the fact that a variation in any part of a complicated organ will, in itself, disturb the harmonious adjustment of other parts, and will thus cause them to throw off gemmules, and thus to induce variability in the next generation.
The fact that change is needed in any part will be the cause of variation in this part, and the time which is needed to restore all parts of an organ to a position of equilibrium will thus be almost infinitely reduced. The argument of those who hold that life has not existed upon the earth long enough for the evolution of all the adaptations of nature by the selection of fortuitous variations will thus lose all its weight.
Darwin believes that the evolution of wild species is due, like the formation of many domesticated races, to very slow modification by the natural selection of great numbers of very slight and inconspicuous variations, but many other authors have given reasons for believing that this is not the case.
Many of our most peculiar domestic races have originated suddenly, and there are reasons for believing that the history of the evolution of each species is divided into periods of abrupt and extensive modification, alternating with periods of comparative stability. This subject, like those which have been briefly noted in the last two sections, will be fully discussed in Chapter XL, and I will only dwell upon it long enough at present to point out that our view of the cause of variation implies that any particular change should in itself be a fruitful source of still greater modification, so that as soon as a tendency to vary becomes established it will continue to increase until an equilibrium is again established by the natural selection of those modifications which are adapted to the environment.
Correlated Variation. This subject will be fully discussed in the chapter on homology, but a few words upon it will not be out of place here.
Darwin, who frequently uses the term, includes under it facts which belong to two somewhat different classes. When any part varies, the organs with which it is most directly associated also tend to vary in such a way as to restore the harmonious adjustment between the various parts: and a variation in one part is often accompanied by variation in homologous parts.
These two cases shade into each other somewhat, hut it will be convenient to treat them separately. The first has just been briefly examined, p. 156, and what follows relates only to the second class of cases—the variation of homologous parts.
The most familiar illustration of this law is the fact that in most bilateral organisms homologous parts on ho'h sides of the body tend to vary together. The law holds in radially symmetrical organisms also. All the petals of a regular flower generally vary in the same manner, but there are many exceptions.
The front and hind limbs of vertebrates tend to vary in the same manner, as we see in long and short legged or in thick and thin legged races of horses and dogs.
It is stated that when the muscles of the arm depart in number or arrangement from the proper type they almost always imitate those of the leg, and so conversely the varying muscles of the leg imitate the normal muscles of the arm. There are many cases where a parent with extra fingers has produced a child with extra toes, or the reverse, and in other cases a parent with only one extra digit on one hand has had children with supernumerary digits on both hands and both feet.
In certain pigeons and fowls, especially in the trumpeter pigeon, long feathers, like the primary wing feathers, grow on the outside of the leg and on the two outer toes, and in pigeons with the feet thus feathered the two outer toes are partially connected by skin, thus showing a marked anatomical resemblance to a wing.
The various appendages which are formed from the skin, such as hoofs, horns, hair, feathers, teeth, etc., are homologous organs, and it is interesting to notice how frequently a peculiarity in one of these structures is associated with similar peculiarities in others.