ABSTRACT
The fauna was changing in composition constantly throughout Pleistocene: new species were introduced by local evolution or immigration, while old species vanished through extinction or by evolving into other species. The most useful measure of faunal change would seem to be an average of the origination and extinction rates. This may be termed the rate of faunal turnover and in this case it is twenty per cent. Analyses of the turnover rate have been made for all the stages and substages recognized in the species list; that is to say, Villafranchian was divided into six substages beginning with the Etouaires phase and ending with the Tiglian, and so on. The Cromerian faunal stratum is of special interest because it divides the Pleistocene into two approximately equal parts as far as the amount of evolution is concerned. It makes up about thirty six per cent of the living fauna and almost the same amount, thirty three per cent, of early Villafranchian.
