272 ANIMAL MORPHOLOGY 



Hilgendorf and Hyatt studied the different layers of the tertiary 

 lake of Steinheim in Wurtemberg. They recognized that certain 

 forms of Planorbis differing little among themselves in the deep 

 layers (the oldest) * are separated little by little from one another, 

 and finally, in the most recent layers, constitute species as reliable 

 as any of those described in this genus of mollusks. 2 Is this a work 

 of pure contemplation and description? Is it not manifest that 

 the authors have reconstructed in their thought a gigantic experi- 

 ment, and if they have not in their power the complete determination 

 of this experiment, do they not at least possess sufficient elements 

 to infer the evolution of the forms without determining the factors 

 of this evolution other than the time-factor, the action of which is 

 unexceptionable in this instance? 



Clearer and still more evident and in any case more in line with 

 current ideas is the application of the experiment in the study of 

 the Lamarckian or primary factors of evolution (cosmical factors, 

 ethnological, etc.) 3 



Indeed it is especially by a return to the ideas of Lamarck that 

 transformationism should cause morphology to progress more 

 rapidly in the experimental path. 



Certainly the conceptions of Darwin were in many respects justi- 

 fied by experiment, even in the strictest sense of the word, and 

 Darwin has proved it himself by his beautiful investigations con- 

 cerning self- and cross-fertilization and concerning climbing plants 

 and carnivorous plants, etc. But it is necessary to recognize how 

 many experimental verifications relative to natural selection, to 

 heredity, demand conditions rarely realized, a length of time which 

 renders them easy of accomplishment only by a group of persons 

 (societies of scholars), or necessitate large resources which most 

 investigators cannot command. 



Apart from some brilliant exceptions concerning whom we shall 

 have occasion to speak later, the disciples of Darwin who have 

 followed most closely the tendencies of their master have under- 

 stood experimentation in the very large sense that we give to this 

 word as applied to a great number of investigations relative to 

 secondary factors. 



The importance of the study of primary factors in evolution did 

 not escape Darwin, but, excellent observer though he was, he was 



1 The four oldest forms were the uncertain varieties of the same species: 

 Planorbis laevis. 



2 The Genesis of the Tertiary Species of Planorbis at Steinheim, Proceedings of 

 the Boston Society of Natural History, 1880; and Transformations of Planorbis at 

 Steinheim, American Naturalist, 1882, p. 441; also Stearns, Proceedings of the 

 Academy of Natural Sciences, Philadelphia, 1881. 



3 To convince us of this, it is only necessary to examine the two beautiful 

 volumes recently published by C. B. Davenport under the title, Experimental 

 Morphology (New York, 1897-99), in which we shall find an excellent resume of 

 what has thus far been attempted in the study of the primary factors. 



