Page:The Scientific Monthly vol. 3.djvu/326

 3«o TEE SCIENTIFIC MONTHLY

types like Qie CntBtacea have annatare for the triple purposes of defence, oSence, aad locomotioD ; they are adapted to less swift motion and include ti»e slowly-moving, bottom-living, armored types of tri- lobites. Then there are slowly moving fixed forms, such as the brachio- pods and gastropods, with very dense armature of phosphate and car- bonate of lime. Finally, there are pelagic or floating types such as the jelly fishes which are chemically protected by the poisonous secretions of their " sting-cells."

There is abundant evidence that in pre-Cambrian time certain of the invertebrates had already passed through primary, secondary, and even tertiary phases of adaptation.

Our first actual knowledge of such adaptations dates back io the pre-Cambrian and ia afforded by Walcotfs discovery" in the Qreyson shales of the Algonldan Belt Series of fragmentary remains of that problematic fossil, Beltina danai, which he refers to the Merestomata and near to the Eurypterids, thus making it probable that either Eu- rypterids, or forms ancestral both to trilobites and Eurypterids existed in pre-Cambrian times. More extensive adaptive radiations are found in the Lower Cambrian life zone of Olenellus, a compound phase of tiilobite evolution representing the highest trilobite development These animals are beautifully preserved as fossils because of their dense chitinous armature which protected them and at the same time ad- mitted of considerable freedom of motion. The relationships of these animals have long been in dis- pute, but the discovery of the ventral surface and append- ages in the Mid-Cambrian Neolenus serratus seems to place the trilobites definitely as a sub-class of the Crus- tacea, with affinities to the existing freely swimming, pelagic pbyllopods.

A most significant biolog- ical fact is that primitively armored and sessile brachio- pods of the Cambrian seas have remained almost un- changed generically to the present time, namely, for a period of nearly thirty million years. These animals afford a classic illustration of the rather exceptional condition known to evolutionists as "balance," re- sulting in absolute stability of type. One example is found in Lin- gulella (Lingnla), of which the fossil form, LinguMla acuminata, characteristic of Cambrian and Ordovician times, is closely similar to !t Walcott, Charles D., 1899, pp. 235-244.

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