F. R. Coicper Reed— Blind TriloUtes. 559 



we know prevails at great depths, and which with considerable 

 probability is held to have existed in early geological times over 

 wide areas owing to the non-differentiation of climatic zones. 



It is also deserving of mention that the very character of the beds 

 which accumulated in the Lower Cambrian Period in Wales, where 

 so many of the blind genera of our Group 1 are found, points to 

 rapidity of deposition and fairly shallow water. ^ 



Some members of our * adaptive ' group of blind trilobites 

 probably lost their eyes as a result of their burrowing habits in 

 the soft ooze on the sea-floor, or from frequenting muddy turbid 

 waters in which organs of vision would be of little or no service, as 

 Barrande suggested for the Bohemian forms in the Ordovician shales. 

 Others may have inhabited submarine caves into which few i-ays of 

 light could penetrate, and natural selection may have developed 

 other organs of sense of greater use to them than eyes. A few may 

 have generally lived in the deep-sea abysses, and occasionally have 

 visited the shallower regions, in which their remains were entombed 

 in the deposits there accumulating. Others may have lived in 

 shallow water, but possessed delicate tactile organs and more highly- 

 developed other sensory powers as a compensation for the absence 

 of eyes. Others may have possessed nocturnal habits. If it be difficult 

 to account for the blindness of many living shallow-water Crustacea 

 we ought not to be surprised if we find greater difficulty in 

 discovering any satisfactory explanation of some fossil forms. But, 

 as above suggested, there are several ways of reasonably accounting 

 for the absence of eyes, and we need not despair of ultimately 

 solving each difficult case. 



Conclusion. 



From the above examination into the nature and distribution of 

 blind trilobites we hav^ seen that the great majority of them are 

 primitive forms, possessing no compound eyes because of their low 

 phylogenetic rank and great geological age. No question of adaptation 

 to environment therefore concerns these blind forms. But there is 

 a small number of blind trilobites which we ai-e led to believe 

 owe their blindness to degeneration of the visual organs as a result 

 of certain conditions of life. From the physical and biological 

 evidence of their occurrence we must suppose that different factors 

 have produced the same result. We have seen that amongst modern 

 Crustacea blindness is a condition brought about by a variety of 

 causes, and is by no means always dependent on their bathymetrical 

 distribution or aphotic habitat. Such appears also to have been the 

 case in the geological past, and each instance must be considered on 

 its own merits and with due attention to circumstantial evidence. 



' Hicks, Pres. Add., Proc. Geol. Soc, 1897, vol. liii, p. 68. 



