OF PLANORBIS AT STEINHEIM. 5 



such points a? these are seized upon by one or the other of the contending parties, 

 and magnified into importance. Short as my stay was, it was quite sufficient for the 

 gathering of bags of sand, each containing hundreds of specimens from every layer which 

 I saw, so that the positive flicts stated were proved, some of them by repeated instances. 

 This account is given principally in order to enable others to judge as nearly as possible how 

 much weight theoretical opinions may have had in governing my I'esiilts. For the same 

 reason also, I have preferred photographs to drawings. The distinguished draughtsman, 

 the late Mr. Sonrel, to whom Prof. Agassiz and others owed so many of their most 

 beautiful j^lates, has assisted me by his advice, and has photographed the first three plates 

 in a manner which will be appreciated by all who have attempted to deal with such 

 difficult subjects. The remaining plates were made by Mr. Black of Boston, who took the 

 greatest pains to produce good results. The shells were mounted upon pieces of slate 

 with cement, and then enlarged by the camera. This, though not large enough in the 

 first three plates to show all the characteristics of many of the smaller forms, is still 

 sufficient for the immediate purposes of this memoir. The remaining plates are on a 

 larger scale, and give the separate series and their theoretical relations more in detail. 

 These contain true PI. levis, from Undorf, sent me by Prof Dr. Sandberger, and the 

 closeness of the resemblances between these and the pit forms is thus shown. Those who 

 cannot credit the evolutionary hypothesis, are advised to try to sejDarate these out from the 

 rest of the forms on plates 4-7, without previously consulting the names of the species, 

 and then to compare results with the descriptions of the plates. 



Useless repetition has been avoided, and the nine hundred and fifty-three specimens 

 photographed, and twenty-eight draAvn with the camera-lucida, a total of nine hundred and 

 eighty-one, do not by any means exhibit all the variations. Each one was selected 

 after having been handled, examined, and classed with its congeners many times. The 

 principal varieties were all previously drawn by myself, with a camera especially 

 constructed for the purpose, before the present plan of figuring by the wholesale 

 was thought of, but none of these are reproduced in the plates. 



In spite of previous experience, I had hoped to find a perfect demonstration in 

 the concrete of the theory of the transmutation of species. That I was rightly and 

 legitimately disappointed in this, I have endeavored to point out in the chapter on 

 the geology of Steinheim. The Pit deposits certainly do exhibit the fullest, and perhaps 

 one of the most complete series of genetically connected forms, which it is perhaps 

 possible to obtain, but there is here, as well as in the adjacent limestones and in those on 

 the rim of the basin, a deficiency of data, which no explorations can make absolutely 

 perfect. It is my wish to be here fully understood, not as meaning that there is any 

 deficiency of observable facts. On the contrary, the varieties ai'e so abundant, that it 

 becomes difficult for the impartial investigator to avoid becoming hopelessly confused, but 

 notwithstanding this excess of riches, the record is and must ever remain exceedingly 

 incomplete. An infinitude of details are necessarily absent, the animals themselves must 

 ever remain unknown, and we are forced here as elsewhere to construct our genetic 

 tables upon theoretical grounds, which must necessaiily change from day to day as 

 knowledge progresses. 



