« 이전계속 »
consider the great obscurity and complexity of the case, and the great difficulty in conducting rigid experiments, the balance of the evidence from hybrids seems to be greatly in favor of our view of the nature of heredity. It certainly presents features which are inexplicable in any other way, and perfectly simple and natural if our view is accepted.
THE EVIDENCE FROM VARIATION.
Causes of variation—Changed conditions of life induce variability—No particular kind of change is necessary—"Variability is almost exclusively confined to organisms produced from fertilized ova—Bud variation very rare—History of the Italian orange—The frequency of variation in organisms produced from sexual union, as compared with its infrequency in those produced asexually, receives a direct explanation by our theory of heredity—Bud variation more frequent in cultivated than in. wild plants—Our theory would lead us to expect this—Changed conditions do not act directly, but they cause subsequent generations to vary —Tendency to vary is hereditary—These facts perfectly explicable by our theory—Specific characters more variable than generic—Species of large genera more variuble than those of small genera—A part developed in an unusual way highly variable—Law of equable variation—Secondary sexual characters variable—Natural selection cannot act to produce permanent modification unless mauy individuals vary together—Our theory is the only explanation of the simultaneous variation of many individuals—This theory also simplifies the evolution of complex structures—Saltatory evolution—This is explained by our theory of heredity —Correlated variation of homologous parts—Parts confined to males more variable than parts confined to females— Males more variable than females—Summary of last two chapters.
Hie Causes of Variation.
Certain authors have held that variability is a necessary accompaniment of reproduction; that it is determined by something within rather than without the organism, but Darwin, after long and careful study of the subject, reaches the conclusion that each variation is excited by a change of some kind in the environment. It is impossible to expose animals for any length of time to absolutely uniform conditions, and we therefore find that when careful attention is given to the subject, minute individual differences may be detected in animals which are apparently most uniform. A shepherd easily learns to recognize each sheep in a large flock, and ants are able to perceive a difference between the members of their own community and those from another nest.
It is impossible to show by direct proof that uniform conditions of life would prevent variation; but it is quite possible to approach the subject from the other side, and to show that slight external changes cause slight variability, and greater changes greater variability.
Wild animals and plants vary somewhat and have individual peculiarities, for each one is under slightly different relations to the external world from all the others, but as compared with domesticated species their conditions of life are very uniform.
A wild animal has become habituated to the circumstances under which it lives, by exposure, for generations after generations to the action of natural selection, and a host of competing animals tend to keep it in its place, but domesticated animals are protected from their enemies and competitors, they are removed from their natural conditions, and they are frequently carried from their native land and are exposed in other countries to unnatural food and climate. They are compelled to change their habits, and they are never left long at rest, or exposed for any considerable length of time to closely similar conditions, but they are carried from district to district, and their food and treatment varies considerably.
We accordingly find that, with few exceptions, all our domesticated animals and plants vary more than their wild relations. Even the goose, one of the least variable of domesticated animals, varies more than almost any wild bird, and according to Darwin, hardly a single plant can be named, which has long been propagated and cultivated by seed, that is not highly variable.
These considerations force us to conclude that variability is not a necessary contingent of reproduction, but that the production of the gemmules which give rise to variation is excited by changes in external conditions, and we must agree with Darwin that "it is probable that variability of every kind is directly or indirectly caused by changed conditions of life; or to put the case under another point of view, if it were possible to expose all the individuals of a species during many generations to absolutely uniform conditions of life, there would be no variability."
When we come to examine the effect of different conditions of life we find that we cannot attribute the variability to one rather than the other. The essential thing is change, but not any particular kind of change.
Variation is frequently caused by a change of climate, but this is by no means essential, for most cultivated plants yield more varieties when cultivated in their native country than when removed to other climates. (Darwin, Variation, ii. p. 310.)
Change of food is often a cause of variation, but that this is not necessary is shown by the fact pointed out by Darwin, that fowls and pigeons are the most variable of domesticated animals, although their food is nearly the game as that of their wild allies, but is much less varied tlian that which they would find for themselves in a state of nature.
Excess of food often causes variation, yet the turkey and goose have been encouraged and tempted for generations to feed to excess, and they have varied but little.
These examples show that the character of the change is unimportant, and that variability cannot be attributed to the exclusive influence of any particular class of external conditions; that the exciting cause of variation is change, but not any particular kind of change.
Darwin quotes a number of cases to show how slight a change may result in variability.
Thus the wild horses of the pampas of South America are of one of three colors, and the wild cattle are of one color; but when the same horses and cattle are domesticated, although they are not confined, but are allowed to run at large like the wild forms, they entirely lose their similarity of color, and display the greatest diversity in tliis particular. In India several species of fresh-water fishes are reared in great tanks as large as natural ponds, and they are all very variable. Darwin quotes from Downing the statement that varieties of the plum and peach which breed truly by seed, lose this power, and like other worked trees give variable seedlings when grafted on another stock.
Variability almost Exclusively Confined to Organism Produced from Fertilized Ova.
The only method open to us besides the study of hybrids for observing the influence of the sexes in heredity, is by a comparison of sexual with asexual heredity. As I shall show in another place, all the various forms of asexual reproduction are so connected that we may pass from fission, or the formation of two new organisms