A. H. Clark — Recent Crinoid Faunas. 127 



Aet. XIII. — The Comparative Age of the Recent Crinoid 

 Faunas: by Austin Hobart Clark. 



There is no point of more importance in the study of recent 

 zoogeographic areas than the determination of their relative 

 antiquity. Such a study must necessarily be based upon theo- 

 retical grounds ; but there appear to be certain principles by 

 which we may, if we are sufficiently cautious, be safely guided. 

 These principles, as here stated, have been worked out; chiefly 

 through a study of the crinoids, though they appear to be applic- 

 able to the other marine groups, and probably also to many 

 terrestrial forms. They concern the faunas solely in their 

 physiological phylogenetic aspect, disregarding the external 

 influences of immediate environment. 



It must be borne in mind that faunal values vary very greatly 

 in each class of animals, aud often in the several orders in each 

 class, as a result of varying reactions to physical, chemical and 

 general economic environment. For instance, sessile or fixed 

 animals would in each locality pass through a definite faunal 

 cycle, such as we see indicated by the crinoids. Among pelagic 

 animals, on the other hand, or among animals with a long free- 

 swimming larval stage, faunal areas comparable to those delimi- 

 ted by the crinoids scarcely exist, and where they are indicated 

 faunal cycles cannot be shown with any degree of certainty. 

 This is due to the continual replenishing of areas, either directly 

 or, as it were, by relays of generations, from a distant source, 

 which, at least in some cases, is probably correlated with a 

 back current carrying to the original source the young of ani- 

 mals which for many generations have lived in a distant area. 

 Thus any change affecting the animals in any particular area 

 would be extremely slow in affecting the species as a whole, 

 for it would be dissipated over such an extent of faunal terri- 

 tory that its influence would be largely counteracted before it 

 could acquire any headway. 



Many species inhabiting deep or cold water are recognized 

 as the deep or cold water forms or varieties of other species 

 living in shallow water near shore. It is probable that at least 

 in some of these cases the difference may be caused by physio- 

 logical changes due to a deep or to a cold environment acting 

 upon the developing young of a single generation, something 

 as the color pattern of butterflies is changed by exposure to heat 

 or cold at the time of pupation. If this were true, such 

 deep or cold water species would have no faunal significance, 

 no matter how great the significance of species associated with 

 them might be, for their character and development would not 

 be due to anything inherent within themselves, but would be 

 quite dependent upon the development of the parent species 

 under the more normal conditions. 



