Page:Scientific results HMS Challenger vol 18 part 1.djvu/123

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156. Formation of Species.—The totality of similar forms, which we unite in one species, and which in the earlier dogmatic systems was regarded as a category of absolute value, possesses only a relative value like all other systematic categories (§ 155). According to the individual views of the systematist and the general survey which he has attained of the smaller and larger systematic groups, the conception of a species adopted in his practical work will be wider or narrower. In the present systematic arrangement a medium extent has been adopted. It is shown that in the Radiolaria, as in all other extensive groups of organisms, the constancy of the species is very variable in the different groups. Many families of Radiolaria are very rich in "bad species," i.e., very variable forms, in which the process of the formation of species is seen in progress; such, for example, are—among the, the Sphærozoida, Stylosphærida, Phacodiscida and Pylonida; among the , the Amphilonchida and Phractopeltida; among the , the and ; and among the , the Aulacanthida, Sagosphærida, Castanellida and Concharida. On the