104 LECTUEES ON BIOLOGY 



of chifcin or lime, like the arthropods or echinoderms ; or shells 

 and houses of lime, silicic acid, &c., like snails, mussels, and 

 many species of unicellular protozoans. It is very rare to find 

 animals with soft bodies preserved, or at any rate their forms 

 impressed on stone, as for instance in the case of the famous find 

 of Medusa in the lithographic slates of the Upper Jura, near 

 Solenhofen and Eichstatt. The impressions left by these delicate 

 animals on the hard stone are sometimes so perfect as to make 

 an exact systematic determination possible. But unfortunately 

 such treasure-troves are rare exceptions for which the inaccuracy 

 of the fossil-documents found elsewhere cannot compensate us. 

 Above all, the most primitive forms of life which had probably 

 not yet reached the organization of the cell and would therefore 

 be of particular interest to us have completely and for all times 

 been expunged from the geological record : a fundamental reason 

 why we shall never be able to substitute exact knowledge for 

 the present vague guesses concerning the first appearance of 

 organisms upon our earth. 



Another proof of the inadequacy of the palseontological evidence 

 is that generally only the remains of aquatic animals were 

 preserved in large numbers, but terrestrial animals only excep- 

 tionally in favourable places, that is, where there was a possibility 

 that the animal corpses would be rapidly covered by a crust of 

 mud or earth, and thus saved from destruction. 



Geologically the oldest stratum in which we meet with 

 fossilized life is the Cambrian stage. We see here chiefly 

 quaintly constructed crustaceans, called Trilobites, on account of 

 the tri-partition of their bodies. These ancient animals were 

 thorough cosmopolitans, for they, or related species, have been 

 found in the Cambrian strata of North America, England, and 

 Bohemia. As they cannot be credited with having been able to 

 undertake long journeys we must assume, for the purpose of 

 explaining their world-wide distribution, that they were probably 

 carried by sea and wind in the larval state over enormous spaces. 



One of the most important types of the Cambrian Trilobite 

 fauna is a specimen of the genus Paradoxides (fig. 25, 1 ). The 

 many distinctive features are the divisions of the body into three 

 parts, an impaired median axis, the so-called spindle, and the paired 

 lateral parts or pleurae. The anterior end is formed by a cephalic 



