Note BB — Chapter IV · VARIATION AND NATURAL SELECTION. (continued 2)

Two more technical examples may be noticed in a note. (1) Professor Haeckel has recently (Challenger Reports, vol. xxviii.) shown that the Siphonophora include two groups, closely resembling each other, but of different ancestry: (a) The Disconanthæ, traceable to trachomedusoid ancestors; (b) the Siphonanthæ, traceable to anthomedusoid ancestors like Sarsia. (2) M. Paul Pelseneer has been led to the conclusion that the pteropod molluscs also include two groups resembling each other, but of different ancestry: (a) The Thecosomes, traceable to tornatellid ancestors; (b) the Gymnosomes, traceable to aphysiid ancestors. In each case, the ancestral sea-slug has been modified for a free-swimming life.