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This assumption requires only a limited number of mutative periods, which might well occur within the time allowed by physicists and geologists for the existence of animal and vegetable life on the earth. Summing up the main points of these introductory remarks, I propose to deal with the subjects mentioned above at some length, devoting to each of them, if possible at least an entire lecture.
So for instance, the "red-veins" have never produced any dwarfs, notwithstanding they are produced very often by the parent-type. And in crossing experiments also the red-veins gave proof of the absence of a mutative capacity for their production.
From this discussion we may infer that it is quite possible that a large part of the progressive changes, and a smaller part of the retrograde mutations, are combined into groups, owing their origin to common external agencies. The periods in which such groups occur would constitute the mutative periods.
But the chance of mutative changes in larger numbers is manifestly much reduced by this experiment, and they may be expected to form a very small proportion of the culture. The second question which arose from the above result was this. Could the mutation be repeated? Was it to be ascribed to some latent cause which might be operative more than once?
On the other hand, the hypothesis of mutative periods is by no means irreconcilable with the observed facts of constancy. Such casual changes can be proved by observations such as those upon the evening-primrose, but it is obvious that a disproof can never be given.
The fact of the mutation may be very probable, but the full proof is, of course, wanting. Such is the case with the mutative origin of Xanthium commune Wootoni from New Mexico and of Oenothera biennis cruciata from Holland. The same doubt exists as to the origin of the Capsella heegeri of Solms-Laubach, and of the oldest recorded mutation, that of Chelidonium laciniatum in Heidelberg about 1600.
Considering the mutative period of our evening-primrose as one unit-stride section in the great genealogic tree, this period includes two nearly related, but not identical changes. One is the production of new specific characters in the latent condition, and the other is the bringing of them to light and putting them into active existence.
Such very rich genera however, are not the rule, but are exceptional cases, indicating the rarity of powerful mutative changes. On the other hand, species may remain in a state of constancy during long, apparently during indefinite, ages. Many facts plead in favor of the constancy of species. This principle has always been recognized by systematists.
The faculty of producing nanella or lata remains the same through all the years. This faculty must be one and the same for all the hundreds of mutative productions of the same form. When and how did it originate? At the outset it must have been produced in a latent condition, and even yet it must be assumed to be continuously present in this state, and only to become active at distant intervals.
Genealogic trees are the result of comparative studies; they are far removed from the results of experimental inquiry concerning the origin of species. What are the links which bind them together? Obviously they must be sought in the mutative periods, which have immediately preceded the present one.
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