FIELD INVESTIGATIONS IN 1 92 1 849 



able changes in the combination of the parts. Organisms are forever 

 either striving for improvement in their life processes or they are sub- 

 jected to changing conditions that require modification in their habits 

 and physical characteristics; and these relatively normal modifications, 

 whether acquired through optional or enforced conditions, are supple- 

 mented over and over again by more purely accidental impositions and 

 by peculiarities and tendencies inherited from both immediate and more 

 remote ancestors. 



The Crinoidea would seem to include especially favorable illustrations 

 of the principle. One would expect it to be illustrated particularly by 

 such species of the class as have arms that are frequently and variously 

 divided and at regular intervals, arms that zigzag in some and are nearly 

 straight in others, with variously, but in each species regularly, disposed 

 nodes on their joints, or have variously arranged ridges, nodes, pits, and 

 other features on both the dorsal and ventral surfaces of their calyces. 

 Therefore, if we accept the occurrence of a practically unchanged reap- 

 pearance of the Upper Ohara crinoid fauna in the Renault in Monroe 

 County, Illinois, as an indisputable fact, one can hardly escape the alter- 

 native convictions that either the time that elapsed between the dates of 

 the two crinoid zones — that is, the time consumed in the deposition of 

 the intervening Aux Yases sandstone plus the times represented by the 

 supposed breaks between this sandstone and the formations lying imme- 

 diately beneath and above — was too short to make any appreciable change 

 in this fauna, or the crinoids in this instance maintained their character- 

 istics, both as regards their structural peculiarities and their associates 

 in making the fauna, with uncommon persistence. 



Neither of these conclusions seems satisfactory. The time most prob- 

 ably was long enough to bring about very considerable changes, not only 

 in the composition of the fauna, but also in the characters of the species 

 themselves; and it seems unreasonable and unwarranted by any known 

 facts to assume that these in large part highly specialized crinoids were 

 so much more stable than usual. Under the circumstances, I can not 

 avoid the suspicion that there is something wrong about the age assigned 

 to the Monroe County crinoid bed that contains so many species which 

 elsewhere are found only in upper members of the Sainte Genevieve lime- 

 stone. The only kind of evidence that under my present view as to the 

 essential cotermineity of the Sainte Genevieve in Missouri, Illinois, In- 

 diana, Kentucky, and Alabama I can accept as proving the distinctness 

 of the Monroe County and the Huntsville crinoid zones is the presence 

 of the Aux Vases sandstone beneath the crinoid zone in Monroe County. 



It is a well known fact that the Aux Vases sandstone is not continn- 



