50 Silurian Fossils of Canada, 



stone form abundant in certain localities, but not generally 

 distributed. Thus in the State of New York, accordino- to Pro- 

 fessor Hall "it abounds at Trenton Falls and at Suo-ar River in 

 Lewis County," but "is scarcely known as occuring in the 

 Chaniplaim valley."* In Canada it is found at Lachine and at 

 several other places, but there are hundreds of good exposures 

 of the rock in the Province that have been carefully examined, 

 where not a single specimen has been seen, although in all the 

 localities ;S^. alternata is more or less common. S. camerata 

 occurs at one spot in the vicinity of Ottawa, but I have never 

 met with it elsewhere. ^S'. ienuistriata may be collected in the 

 hard black limestone around the base of the mountain of Montreal, 

 particularly in the neighbourhood of the McTavish monument 

 and also at Ottawa and two or three other places, but does not 

 occur at all in the majority of the localities of the Trenton lime- 

 stone. These three species, therefore, must have been capable of 

 existing in certain places only, on the bottom of the ocean during 

 the period of the accumulation of this rock, while >S'. alternata^ 

 flourished everywhere. Whether or not, therefore, they are to 

 be regarded as distinct species, this much at least seems prob- 

 able, that they were by some peculiarity in their habits or in 

 their organization, unfitted for so wide a range through the seas 

 as that enjoyed by S. alternata. 



The question, whether or not these supposed species are really 

 distinct, cannot be answered until naturalists shall have discovered 

 some general law of life by an appeal to which they may in all 

 cases determine what is a species as distinguished from a mere 

 variety. It is scarcely necessary to state that such a law if it 

 do exist at all may remain unknown to man for ages, and in the 

 meantime nearly all determinations of species from varieties where 

 the forms are very closely related may be regarded as not 

 positively established. The physical geologist is more inter- 

 ested in the results of investigations which show that certain 

 forms are confined to particular geological horizons than in those, 

 whose sole object is to determine the exact zoological rela- 

 tions of such forms. If it be true, for instance, that that parti- 

 cular form of the genus called Strophomena incrassata is con- 

 fined to the limestones lying next under the Trenton in the fossil- 

 iferous series, it makes no difference to the geologist, whether it 



* Palaeontology of New York, vol. 1, page 107. 



