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cultivated forms to be more prolific 2. gemmules than wild species. The fact that bud variation, like ordinary variation, is most common in cultivated forms, seems to show that the tendency to vary is excited in buds, as it is in fertilized ova, by the influence of gemmules which are thrown off by the cells of the body under new or unnatural conditions, and we can easily understand why it should be more frequent where gemmules are abundant than in a form with few gemmules, for the chance in favor of the accidental transmission of a gemmule to a growing or nascent bud will increase as the number of gem mules increases. Changed Conditions do not act directly, but they cause
Subsequent (fenerations to vary. This strange and, as I hope to show, highly significant law has been noted by many observers, and a long list of illustrations might be quoted.
As Darwin points out, it is certainly a remarkable fact that changed conditions should at first produce, so far as we can see, absolutely no effect, but that they should subsequently cause the character of the species to change.
The late Dr. Jared P. Kirtland told me that for more than forty years he tried in vain to obtain varieties from the common red cherry, but that when at last varieties began to appear the variability was rery great: that after it had once become established it continued for many years with no diminution.
It is well known that when new flowers are first introduced into gardens they do not vary, although all, with rarest exceptions, ultimately vary.
Darwin, in his Variation, Vol. ii. p. 316, quotes the following illustrations of this law : “Mr. Salter remarks that every one knows that the chief difficulty is in breaking through the original form and color of the species, and every one will be on the lookout for any natural sport, either from seed or branch; that being once obtained, however trilling the change may be, the result depends upon himself. M. de Jonghe, with reference to pears, says the more a type has entered into a state of variation, the greater is its tendency to continue doing so, and the more it is disposed to vary still fur| ther. Vilmore says that when any particular variation
is desired the first step is to get the plant to vary in any manner whatever, and to go on selecting the most variable individuals, even though they vary in the wrong direction; for the fixed character of the species once broken, the desired variation will sooner or later appear.
Darwin gives quite a list of authorities to show that after English dogs have been bred for a few generations in India they degenerate, not only in their mental faculties, but in form.
According to Bachman, turkeys reared from the eggs of wild ones lose their metallic tints and become spotted with white in the third generation.
It will be seen from the instances which have been given that the number of generations which are exposed to the new conditions before variation is induced varies greatly. In the case given by Dr. Kirtland, fifty years elapsed before variations of the red cherry began to appear. In the case last quoted, variation appeared in the third generation, and Yarrell says that Australian dingos bred in the Zoological Gardens of England, almost invariably produced in the first generation puppies marked with white and other colors.
Sir Charles Lyell mentions that some Englishmen engaged in conducting the operations of the Real del Monte Company in Mexico, carried out with them some greyhounds of the best breed to hunt the hares which abound in that country. It was found that the greyhounds could not support the fatigues of a long chase in this attenuated atmosphere, and before they could come up with their prey they lay down gasping for breath; but these same animals have produced whelps, which have grown up, and are not in the least degree incommoded by the want of density of the air, but run down the hares with as much ease as do the fleetest of their race in this country.
It is interesting to note in this connection that a tendency to vary is strongly inherited independently of the inheritance of any particular variation. Darwin believes that this tendency to vary may be transmitted by either parent, and he says (Variation, ii. 325) it is certain that variability may be transmitted through either sexual element, whether or not originally excited in them, for Kölreuter and Gärtner found that when two species were crossed, if either one was variable the offspring were rendered variable.
We have already pointed out that the crossing of species is in itself one of the most efficient causes of variation, and we can hardly base upon the observations above given the conclusion that variability may be transmitted by either sex.
The fact that changed conditions do not directly produce variation, but cause subsequent generations to vary, is precisely what we should expect, according to our theory: for a change in the environment of an animal or plant must disturb the harmonious adjustment which natural selection has brought about between the cells of its body and their conditions of life. Such a
change, if considerable, could hardly fail to affect certain cells unfavorably; and it would therefore cause the production of gemmules, thus inducing variation in later generations. ! We can also understand how a tendency to vary may be hereditary, for if certain cells of the body vary, they will exercise a disturbing effect upon adjacent or related cells, and these, transmitting gemmules, will hand on the tendency to vary to succeeding generations.
Secondary Laws of Variation. The law that variability is itself hereditary involves a number of secondary laws, all of which find a ready explanation in our theory of heredity.
Among these secondary laws is the law that " specific characters are more variable than generic characters." Darwin has given the evidence of the existence of this law (“ Origin of Species,” p. 122), so it will not be necessary to discuss it, or to do more than point out that the theory of heredity furnishes an explanation of it.
The characters which are common to all the species of a genus, and which distinguish it from other genera, are, as a rule, much older than those which distinguish one species of the genus from the other species. The specific characters or features which distinguish each species of a genus from the others, are features which have appeared as new variations since the time when the various species diverged from the common ancestor from whom they inherit their common or generic characters. As specific characters are of more recent acquisition than generic characters, natural selection will have had less time to act upon the former than upon the latter. The adjustment between a specific character and its environment will therefore be, as a rule, less complete and perfect, and the cells which are involved will therefore have a greater tendency than those involved in generic characters to throw off gemmules. These characters will therefore be more variable in the descendants than generic characters.
Another law, the evidence for which is given by Darwin on page 44 of the “Origin of Species,” is that “species of the larger genera in each country vary more frequently than the species of the smaller genera.”
When a country contains a great number of species of a genus it is generally safe to conclude that they have recently varied and diverged from each other. As the tendency to vary is in itself hereditary, and as one variation is in itself a cause of other variations, our theory of heredity would lead us to expect species which have recently undergone considerable change to show a tendency to vary still further, and we should therefore expect the species of large genera to be, as a rule, more variable than the species of small genera, although there is no reason why this rule should be absolute.
A still more interesting law is that “a part developed in any species in an extraordinary degree or manner, in comparison with the same part in allied species, tends to be highly variable” (“ Origin of Species,” p. 119).
When one species of a genus agrees with the other species in most particulars, but differs from them all in some one respect, we may conclude that the peculiar organ or feature has recently been modified. Natural selection has therefore had less time to perfect the adjustment between this part and the remainder of the body than it has had to perfect the relations between other parts, or between the same parts in the other species.
This peculiar part will accordingly be in a favorable state for the production of gemmules, and it will there