F. R. Coivper Heed — Blind Trilobites. 553 



limestones) which accumulated in comparatively clear and pure 

 water. On the other hand, in the British Isles, Sweden, and Russia, 

 when once we get above the Cambrian, there are just as many 

 blind genera in our Group 1 occurring in calcareous beds (e.g. the 

 Keisley and Kildare Limestones, the Leptcena Limestone, and the 

 Eussian homotaxial limestones) as in shales and slates ; and, in fact, 

 some of the blind genera seem restricted to these limestones 

 (Tiresias, Isocolus). The character of the sediment seems also to 

 have but little to do with the presence or absence of the ' adaptive ' 

 species in our Group 2, for while in Bohemia Barrande notices that 

 the blind species of lUmnus occur in the fine ai'gillaceous slates, the 

 northern species of the same genus occu-r principally in the above- 

 mentioned limestones. 



Barrande (loc. cit.) also lays stress on the fact that the large-eyed 

 genera JEglina and Eemopleurides only are found in the two bands 

 of fine argillaceous shale, Ddl, J)d5, in which occur the majority 

 of the blind forms of the Second Fauna; and that in the 

 Primordial fauna of Ktage C an analogous state of things exists in 

 the association of the large-eyed Paradoxides and Hydrocephalus with 

 the eyeless forms. In both cases he imagines the same cause to 

 have been operative, i.e. the turbid muddy state of the water, which 

 made eyes of a normal size useless and led either to the development 

 of enormous visual organs or to the degeneration and loss of those 

 of an ordinary character. Whatever connection, however, these cases 

 may indicate between the association of blind with large-eyed forms, 

 it cannot apparently have been one universally induced by such 

 a cause as Barrande supposes, for we find the large-eyed and blind 

 forms of our Group 1 occurring together in calcareous beds, such as 

 the Keisley and homotaxial Limestones, in which the conditions 

 of deposition must have been of an utterly opposite character. The 

 difference in the facies of the faunas associated shows that the 

 bionomic conditions must also have been dissimilar. 



In connection with the question of the action of the environment 

 on the structure or sense-organs of animals, we may refer to 

 Neumaj'r's^ arguments in favour of regarding the Cambrian fauna 

 as indicating conditions analogous to those now existing in the deep 

 sea, especially as he makes so strong a point of the occurrence 

 of blind trilobites. After the foregoing careful consideration of the 

 subject, it will only be necessary to sketch briefly the kind of answers 

 which may be made to his arguments, and the different explanations 

 possible of the phenomena which he adduces. He mentions four 

 peculiarities of the Cambrian fauna which make him believe that 

 it represents deep-sea conditions : — 



(1) Its wide distribution; but we have remarked above that this 

 merely indicates uniform conditions of life, and that such most 

 probably existed in early geological times. 



(2) Its impoverished character as a fauna, shown by its pover' 

 in variety of types, since it consists chiefly of trilobites 



1 Erdgescliiclite, toI. ii, p. 51. 



