116 



State of New York, according to Professor Hall, " it abounds at Trenton 

 Falls and at Sugar River in Lewis County," but "is scarcely known as 

 occurring in the Champ] ain 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. tenuistriata may be collected in 

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

 particularly in the neighborhood of the McTavish monument, and also at 

 Ottawa and two or three other places, but does not occur at all in the ma- 

 jority of the localities of the Trenton limestone. These three species, there- 

 fore must have been capable of existing in certain places only, on the bot- 

 tom 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 probable, that 

 they were by some peculiarity in their habits or in their organization, un- 

 fitted 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 interested in the 

 results of investigations which show that certain forms are confined to par- 

 ticular geological horizons than in those, whose sole object is to determine 

 the exact zoological relations of such forms. If it be true, for instance, 

 that that particular form of the genus called Strophomena incrassata is 

 confined to the limestones lying next under the Trenton in the fossiliferous 

 series, it makes no difference to the geologist, whether it be in reality a 

 distinct species of itself or only a variety of some other species. Its value 

 to him as a guide, while tracing out the geographical distribution of these 

 Rocks, is not at all affected by the zoological question. In demonstrating 

 the physical structure of the country, he can reason upon varieties with as 

 much safety as upon true species, provided that such varieties are confined 

 to and consequently characteristic of particular portions of the geological 

 edifice. It is therefore, of great importance, not only to ascertain to what 



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



