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 dif- 

 ferences 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 con- 

 tinuous 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 record-, 

 ing the varieties . . . which accompany a definable type 

 ... in the geographical district in which the latter is indige- 

 nous 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 recog- 

 nized 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 



[121] 



