. The origin and evolution of life, on the theory of action, reaction and interaction of energy. preserved asfossils because of their densechitinous armature, whichprotected them and at thesame time admitted of con-siderable freedom of mo-tion. The relationships ofthe trilobites to other in-vertebrates have long beenin dispute, but the dis-covery of the ventral sur-face and appendages in the mid-Cambrian Ncolcnus serratus(Fig. 20) seems to place the trilobites definitely as a subclassof the Crustacea, with affinities to the freely swimming phyl-lopods, which swarm on the surface of the existin
. The origin and evolution of life, on the theory of action, reaction and interaction of energy. preserved asfossils because of their densechitinous armature, whichprotected them and at thesame time admitted of con-siderable freedom of mo-tion. The relationships ofthe trilobites to other in-vertebrates have long beenin dispute, but the dis-covery of the ventral sur-face and appendages in the mid-Cambrian Ncolcnus serratus(Fig. 20) seems to place the trilobites definitely as a subclassof the Crustacea, with affinities to the freely swimming phyl-lopods, which swarm on the surface of the existing oceans. A most significant biological fact is that certain of theprimitively armored and sessile brachiopods of the Cambrianseas have remained almost unchanged generically for a periodof nearly thirty million years, down to the present time. Theseanimals afford a classic illustration of the rather exceptionalcondition known to evolutionists as balance, resulting inabsolute stability of type. One example is found in Lingulella(Lingula), of which the fossil form, Lingulella acuminata, char-. FiG. 20. A Mid-Cambrian ^coloius serratus (Rominger). After Walcott. 122 THE ORIGIN AND EVOLUTION OF LIFE acteristic of Cambrian and Ordovician times, is closely similarto that of Lingiila anatina, a species living to-day. Represen-tatives of the genus Lingula {Lingulella) have persisted fromCambrian to Recent times. The great antiquity of the brachi-opods as a group is well illustrated by the persistence of Lingula(Cambrian—Ordovician—Recent), on the one hand, and ofTerehratula (Devonian—Recent), belonging to a widely differ-ing family, on the other. These lamp-shells are thus charac-teristic of all geologic ages, including the present. Reachingtheir maximum radiation during the Ordovician and Silurian,they gradually lost their importance during the Devonian andPermian, and at the present time have dwindled into a rela-tively insignificant group, members of which range fro
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