126 CLARK : BATHYMETRICAL DISTRIBUTION OF CRINOIDS 



there is possible an unbroken organic continuity extending 

 through long periods of geological time, as in the case of the 

 elasipod holothurians, certain anemonies, and many annelids, 

 known only from the Cambrian and from the recent seas; and 

 (2) it fails to emphasize the significance of the gradual differen- 

 tiation in the conditions 

 of marine life as a result 

 of which many organ- 

 isms , originally living 

 together under the same 

 oecological surroundings, 

 have during geological 

 time travelled gradu- 

 ally, and increasingly, 

 diverging paths, so that 

 now they have become 

 widely separated from 

 their original compan- 

 ions, like the phyllopod 

 crustaceans, certain 

 marine worms, and the 

 elasipod holothurians, all 

 of which lived side by 

 side in the Cambrian 

 seas. 



As yet we have not 

 sufficient information at 

 hand to permit us to 

 state with certainty that 

 we' shall ever be able to determine time factors of palseonto- 

 logical value from the study of the recent marine animals alone; 

 but we have enough data to be able tentatively to suggest cer- 

 tain lines of procedure by which it may be possible in the future, 

 when our knowledge of the present marine fauna is more detailed, 

 to classify with more or less accuracy the various animal groups 

 according to their comparative geological antiquity. 



In the following lists are given for the subfamilies and higher 

 groups of recent crinoids: 



Fig. 1. Difference between the average range 

 in depth, expressed as a percentage of the total 

 range, and the average depth of habitat, ex- 

 pressed as a percentage of the mean depth of 

 habitat, together with the average range in 

 depth expressed as a percentage of the average 

 depth of habitat. 



