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In order to meet this difficulty we must recognize two sorts of species. The systematic species are the practical units of the systematists and florists, and all friends of wild nature should do their utmost to preserve them as Linnaeus has proposed them. These units however, are not really existing entities; they have as little claim to be regarded as such as genera and families.
All the instances of the colonel, in behalf of a charming little girl, four years old, whose mother and sister had already taken the infection, could get from her father nothing more than a promise "to think of it! If it's her fate " said he. Perhaps some ingenious systematists might hence deduce a fresh argument in favour of the alleged oriental origin of the Irish.
This is clearly shown by hybrids never having been raised between species ranked by systematists in distinct families; and on the other hand, by very closely allied species generally uniting with facility. But the correspondence between systematic affinity and the facility of crossing is by no means strict.
The extreme application of the principle would no doubt disturb the limits between many species and varieties as now recognized. It is not to be forgotten however that all taxonomic distinctions, which have not been confirmed by physiologic tests are only provisional, a view acknowledged by the best systematists.
After getting them aboard, the canoe was cleared of the bushes; and the paddles being once more called into service, the little craft shot down-stream like an arrow. Lucien now continued his "monograph" of the American ducks. "There are," said he, "more than two dozen species of ducks on the waters of North America. These the systematists have divided into no less than eighteen genera!
Many systematists have been so strongly impressed with its absolute constancy and its behavior as an ordinary species, that they have elevated it, as it is called, to the rank of a species.
The ablest systematists prefer to consider the ordinary species as collective groups, calling their constituents "The elements of the species," as was done by A.P. De Candolle, Alph. De Candolle and Lindley.
The hybrids of Janczewski were quite fertile, and he points out that there is no good reason why they should not be considered as good new species. If they had not been produced artificially, but found in the wild state, their origin would have been unknown, and there can be no doubt that they would have been described by the best systematists as species of the same value as their parents.
Wallace or when analogous views on the origin of species are generally admitted, we can dimly foresee that there will be a considerable revolution in natural history. Systematists will be able to pursue their labours as at present; but they will not be incessantly haunted by the shadowy doubt whether this or that form be a true species.
All of these three rules are supported by a large amount of evidence. The complete fertility of varietal hybrids is so universally acknowledged that it is not worth while to give special instances. With many prominent systematists it has become a test between species and varieties, and from our present point of view this assumption is correct.
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