534 GEOLOGY 



were doubtless many species not now known. The defensive in- 

 vestitures of the lower forms, not fully accounted for by the known 

 Cambrian species, are now much more nearly explained by the 

 prevalence of cephalopods and the presence of fishes. The armors 

 of these dominant forms may have been defensive against their own 

 kind. The fact that vegetal and animal tissues are not represented 

 among fossils, save in exceptional cases, probably signified that 

 the bacteria concerned in the decomposition of organic matter were 

 abundant. 



Ecological, social, and mental development. It seems clear that 

 the adaptation of the various forms of life to one another and to 

 their physical environment had reached a higher stage of adjustment 

 than in the Cambrian, an adjustment not greatly inferior to that 

 which now prevails among the corresponding orders. It is not 

 improbable that the mental development also approached some- 

 what nearly that now possessed by corresponding types. Higher 

 types within the same orders have been developed since in many 

 cases, and probably higher mental functions; but some of the 

 Ordovician forms have since suffered degeneration. The Ordovician 

 ancestors of the barnacle, for example, a free-moving, active form, 

 was doubtless superior to his sessile descendant of ill-repute. The 

 sum total of ecological adaptation and of social and mental develop- 

 ment, on the average, seems to have advanced with each era. 



The Record of Land Life 



Plants. There are strong theoretical reasons for believing that 

 land plants abounded, but only a few relics doubtfully interpreted 

 as land plants have been found, and they reveal but little. 



Insects. The oldest relic of insect life now known is a rather 

 obscure wing found in the graptolite shales of the Upper Ordovician 

 of Sweden. It is referred to the order of Hemiptera (bugs). Not 

 enough is preserved to show fully the nature of the insect, but the 

 existence of any flying insect of this sort implies the presence of 

 vegetation, and of atmospheric conditions suited to active, air- 

 breathing organisms. 



Succession of faunas. There was a succession of Ordovician 

 faunas, somewhat unlike one another, just as there was a succession 



