FUNDAMENTAL RELATIONS OF ANIMALS 53 



Similar series, though less conspicuous and more limited, may be 

 traced in every class of the animal kingdom, not only among the living 

 types, but also among the representatives of past geological ages, 

 which adds to the interest of such series in showing that the combi- 

 nations include not only the element of space, indicating omnipres- 

 ence, but also that of time, which involves prescience. The series of 

 Crinoids, that of Brachiopods through all geological ages, that of the 

 Nautiloids, that of Ammonitoids from the Trias to the Cretaceous 

 formation inclusive, that of Trilobites from the lowest beds up to 

 the Carboniferous period, that of Ganoids through all formations; 

 then again among living animals in the class of Mammalia, the series 

 of Monkeys in the Old World especially, that of Carnivora from the 

 Seals, through the PlantigTades, to the Digitigrades; in the class of 

 Birds, that of the Wading Birds, and that of the Gallinaceous Birds; 

 in the class of Fishes, that of Pleuronectidas and Gadoids, that of 

 Skates and Sharks; in the class of Insects, that of Lepidoptera from 

 the Tineina to the Papilionina; in the class of Crustacea, that of the 

 Decapods in particular; in the class of Worms, that of the Nudibran- 

 chiata or that of the Dorsibranchiata especially; in the class of Cepha- 

 lopoda, that of the Sepioids; in the class of Gasteropoda, that of the 

 Nubibranchiata in particular; in the class of Acephala, that of the 

 Ascidians and that of the Oysters in the widest sense; in the class of 

 Echinoderms, those of Holothurias and Asterioids; in the class of 

 Acalephs, that of the Hydroids; in the class of Polyps, that of the 

 Halcyonoids, of the Atraeoids, etc., etc., deserve particular attention, 

 and may be studied with great advantage in reference to the points 

 under consideration. For everywhere do we observe in them, with 

 reference to space and to time, the thoughtful combinations of an 

 active mind. But it ought not to be overlooked that, while some types 

 represent strikingly connected series, there are others in which noth- 

 ing of the kind seems to exist and the diversity of which involves 

 other considerations. 



