OF PLANORBIS AT STEIFHEIM. 9 



It seems, therefore, no improbable assumption that the four series, if they answer all 

 the requirements, are natural series, and that the living forms they represent, once had 

 the relations here approximately depicted, and that, though the relationship in time has 

 not yet been cleared up, it is probable that these forms did originate from each other in 

 the succession assigned to them in plate 9.^ It will be understood by all intelligent 

 readers, however, that this, like all other arrangements, is an intellectual generalization 

 from the facts and only, as stated above, an approximation to the natural order in which 

 the animal probably occurred, or was evolved. For example, the diflferent series did 

 not occur as they are placed upon the plate. If the geological succession had been 

 confirmed as first laid down by Hilgendorf, then the true relations in time of the diflferent 

 forms would have been given. Thus instead of placing artificially the representative 

 forms, nos. 10 and 5, on the same level as is here done with these- and others, they would 

 have probably been on different levels, representing corresponding dijBferences in the 

 time or formation in which they occurred. The representation or similarity of form, 

 however, would have remained unchanged, and all the deductions here drawn from such 

 comparisons. 



I make no pretence of originating this method, nor can, so far as I know, any one else. 

 It has grown with the science of Natural History, and nearly every naturalist uses it more 

 or less, whether he recognizes the ultimate meaning of gradations in their serial arrange- 

 ments, or ignores them. 



Theoretically, then, the normal primordial form PI. levis can be considered as having 

 had four varieties before its migration into the Steinheim lake, and as having subse- 

 quently reproduced these, or their immediate descendants in this new field. These are as 



drawn on pi. 9, Fl^Sr, %• 16, PZ-T^^^r, %• 12, Pl°XvT%&g- 8, and PZ. «"t?l?^"', fig- 1- 

 These four principal series, shown on pi. 9, and numbered in sequence 1-7, 8-11, 

 12-15, 16-28, were developed from these four varieties after their migration into 

 the Steinheim lake. While the original forms on the first line had the closest 

 relationship with each other, their descendants gradually diverged, until finally no 

 hybrids connected the diflferent series with each other. This is a somewhat bold asser- 

 tion to make with regard to such closely allied animals as these must have been, and 

 it may possibly be forced to give way to more complete evidence than that at pres- 

 ent possessed by the writer. It cannot, however, be refuted except by an absolutely 

 perfect series of intermediate forms. Tested by the ordinary methods of comparison 

 pursued, especially by paleontologists, every one of these forms are connected by hybrids, 

 and the whole presents to the ordinary observer a chaos of similarities and diflferences. 

 The hybridity must appear not only in the adults, but in the absolute identity of the young 



^ Since the above was written I have had the satisfaction ^ This series is divided on the pkte into three sub-series, 



of reading in a late work of Prof. Dr. Neumayer an unre- 16-20, 18 to 21-24, 21 to 25-28. This last sub-series is 



served confirmation of a precisely similar investigation with also divided in the chapters on the " Descriptions of Series," 



reo-ard to the Arietes, a family of Ammonites occurring in into two sub-series, but here these are resolved into one, 



the Lower Lias, which has been treated according to the this plate having been finished before the two sub-series 



same method. Prof. Neumayer followed the ordinary prac- were distinguished. For all the general purposes of discus- 



tical method of tracing the series by the graded resem- sion in this chapter, they can however be treated as a 



blances of the adult forms, and connected the Arietes with united sub-series without confusion, since the difierences 



the predicted ancestral species Fauna d. Untersten Lias, are entirely those which arise from the greater or less prom- 



Abh. d. K. K. Geol. Reichsan'l. Bd. 7, hft. 5. Wien, 1875. inence of the costae or ribs, as they are sometimes called. 



