68 



OKGANIC DEPENDENCE AND DISEASE 



fauna of the Silurian age. There seems no good reason for 

 not interpreting this occurrence, even though it may be iso- 

 lated, as a definite and purposeful fixation. To regard it as 

 casual would be sophistication. With these preliminary 

 evidences then we find the snail-crinoid adjustment well 

 under way very early and clearly developing from a habit 

 of visitation into a set adjustment to vitiation and para- 

 sitism. 



As we enter the Devonian or next succeeding period of 

 geological time, we are confronted at once by an enormous 

 development of the holostomatous shells 

 of the limpet group, known as Capulus 

 or Platyceras, with various generic 

 allies. They are all very closely allied 

 to the species we have already re- 

 ferred to and have the same inheritance. 

 So abundant are these Devonian limpets 

 and so great their variety that it has 

 more than once been proposed to desig- 

 nate the early Devonian age by the name 

 Capulian or Platyceras stage. In a sud- 

 den outburst of evolution these mollusks 

 here reached their climax. Every stu- 

 dent of these shells recognizes the diffi- 

 culty of assigning specimens of them to 

 well-defined species, because the thin- 

 ness of the shell mouth and its easy 

 adjustability in fixation to the irregulari- 

 ties of the surface to which they may 

 attach themselves have caused many 

 differences of expression. Attachment 

 was their habit and we may expect to find them fixed 

 to various sorts of objects, organic and inorganic, when- 

 ever they have been caught alive or in a cemented condition 

 by the burying sediments. We must then scrutinize every 



Fig. 54. Views of the 

 Sihirian crinoid Mar- 

 sipocrinus coelatus 

 (Phillips ) s ur - 

 mounted by, and 

 partly enclosing an 

 attached snail. Wen- 

 lock limestone, Dud- 

 ley, England. (Cour- 

 tesy of Ml'. Frank 

 Springer. ) 



