THE NATURE OF SPECIES
cally impossible. Shell-fish which are so scarce that a museum collection contains only a few representatives are easily divided into species or varieties; but shell-fish such as oysters, which live together in multitudes, are indefinite and uncertain as to species. The same difficulty has been observed with the sea-butterflies, or pteropods, which live in swarms on the surface of the sea and form a large part of the food of whales. In groups where specific variation was slow, or the members were few, or the fossil remains are rare, the differences are so well marked that the delimitation of species presents no difficulty. Organisms, however, that live together in vast numbers and under similar conditions show continuous variation, and though the individuals may be massed around certain centres, the groups grade into one another.
The arrangement of such groups into circuli instead of into species is a fulfillment of Huxley's prediction in 1880 that “The suggestion that it may be as well to give up the attempt to define species and to content oneself with recording the varieties . . . which accompany a definable type ... in the geographical district in which the latter is indigenous may be regarded as revolutionary; but I am inclined to think that sooner or later we shall have to adopt it.”
The artificial nature of species has been generally recognized by working naturalists; but the term species is still retained. Sir Ray Lankester, with his logical consistency, recommends that it should be abandoned; but it has been maintained from tradition and convenience. The abstracts of the papers contributed by Prof. H. L. Hawkins and Dr. A. E. Trueman to a recent British Association discussion on the “Conception of Species” show that their idea of a species is that of the circulus; and so also is the “species-group” of Dr. Bolton among fossil beetles. It is fully time that the term species should be less frequently used, as it is apt to
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