REMARKS ON THE NATURE OP ORGANIC SPECIES. 



BY PROF. JOSEPH LEIDY. 



The now prevailing idea as to the nature of an organic species is widely- 

 different from what it was formerly. Then it was regarded as an independ- 

 ently created form, distinguished by characteristic marks which were 

 within a limited range permanent ; now it is viewed as a peculiar form com- 

 paratively stable in distinctive characters, but descended from others by grad- 

 ual transformation. Could we see all the organic forms which have existed, 

 we would fail to recognize any species, for all, through infinite variation, 

 would blend with one another. The distinction of species at any given 

 time is generally fairly marked and of ready reference, but in many cases 

 also it fails in its application. While we all look at the Horse, Ass and 

 Zebra as distinct species, we do not look on the White Man, the Mongolian 

 and the Negro in the same light. In the inspection of any extensive series 

 of a genus we commonly recognize a number of well-marked species, but 

 closer investigation often reveals intermediate forms which more or less in- 

 validate the distinction. In some large collections of fossil shells of the 

 later tertiary formations, I have repeatedly been struck with evidences that 

 many of our recent species are really the direct descendants of recognized 

 and distinct fossil species. Among several cases in point, my friend Joseph 

 Willcox has recently directed my attention to an instance. He writes : " I 

 submit to you some shells which may interest you, and of which you are 

 at liberty to publish a notice if you think it worth the while." He con- 

 tinues : " The shells are part of a considerable series which appear to illus- 

 trate the transformation or evolution of an extinct form into that of a living 

 species. All, except the recent specimen, were collected by me in a plio- 

 cene bed of South Florida. From an inspection of the series (see plates 

 IX., X.) I think it may be admitted that the fossil Fulgiir contrarius, hereto- 

 fore regarded as extinct, is still living in a modified form as Fulgtir perver- 

 sus. Fig. I, plates IX., X., represents the Florida form oi Fulgur contrarius, 

 the original of which was found in the miocene formation of North Caro- 

 lina, and was described by Conrad in Am. Journal of Science, XXXIX., p. 



