pages, which represent, somewhat broadened, the 

 substance of a Friday evening discourse delivered 

 at the Academy of Natural Sciences of Phila- 



jf& ** v ' ' "'" -i *'-* 



delphia. Brief though the statement is, it is 

 hoped that it may yet be of service to those to 

 whom the more specialized material is not 



readily accessible. 



A. H. 



ACADEMY OF NATURAL SCIENCES, 



Philadelphia, December, 1887. 



