ORIGIN OF THE SPECIFIC TYPE 305 



The Planorhis strata of Steinlieiiu were first investigated, from 

 the point of view of the theory of descent, by Hilgendorf (1866). He 

 described nineteen different varieties, which, as they are all connected 

 in chronological succession with each other, he grouped together 

 under the name of Flanorh'tH inultiforinis. These little freshwater 

 snails are found in millions in the strata of the former lake-basin of 

 Steinheim, and they are arranged in so orderlj^ and regular a manner 

 that two observers, working independently and at different times, 

 succeeded in building up the genealogical tree in almost the same 

 way. According to Alpheus Hyatt, the later investigator, all the 

 forms are derived from one ancestral form, Planorhis Icevis, from 

 which four different series have descended, one of them splitting up 

 again into three subordinate series. 



All the individual members of these series are connected by 

 intermediate forms in such a manner that a long period of constancy 

 of forms seems to be succeeded by a shorter period of tr-ansformation, 

 from which again a relatively constant form arises. 



We see, therefore, that the idea of species is fully justified in 

 a certain sense ; we find indeed at certain times a breaking up of 

 the fixed specific type, the species becomes variable, but soon the 

 medley of forms clears up again and a new constant form arises — 

 a new species, which remains the same for a long series of generations, 

 until ultimately it too begins to waver, and is transformed once 

 more. But if we were to place side by side the cross-sections of this 

 genealogical tree at different levels, we should only see several well- 

 defined species between which no intermediate forms could be recog- 

 nized ; these would only be found in the intermediate strata. 



The problem we have now to discuss is, how it comes about 

 that relatively sharply defined species exist which are connected with 

 ancestral forms further back, but which form among themselves an 

 exclusive, more or less homogeneous, host of individuals. How does 

 it happen that we everywhere find a specific type, and not an endless 

 number of individual forms connected with one another in all directions? 



This would require no further explanation if a phyletic evolu- 

 tionary force impelled the forms of life to vary in a definite manner, 

 and thus to become transmuted into new forms in the course of 

 generations. In that case the whole genealogical tree of the organisms 

 on the earth must have been potentially contained in the lowest 

 moneron, so that, given time and the most indispensable general 

 conditions of existence, the living world just as we know it must have 

 x-esulted. Nageli was the first to express this view, and he followed 

 it out consistently, not even hesitating to deny the existence of all 



II. X 



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