FAREWELL LECTURE _ 265 



After the appearance of Darwin's book, there occurred a great 

 and general change of view in all branches of biology. In fact, 

 outside the great discoveries of Copernicus and Galileo, there can- 

 not be cited another example having so deep an influence on the 

 general opinions of naturalists. Darwin was not the first to con- 

 ceive and pronounce upon the unity of all life; but that he was able 

 to produce stronger proofs and to direct the trend of thought con- 

 stitutes his undying fame. 



In the field of paleontology the consummation of this change did 

 not, of course, go on so simply and, at least with us, so entirely in 

 accordance with the views of Darwin as one is apt to imagine. The 

 Darwinian theory of the variability of species was essentially based 

 on selection and related appearances. Paleontology, however, teaches 

 otherwise. It teaches that the terminology for single divisions of the 

 stratified terranes, characterized by their fossil remains, finds appli- 

 cation over the entire earth. Therefore from time to time there 

 must have occurred, in some way, general changes affecting the 

 entire physical condition of the world. Nor is there seen a per- 

 petual and continuous changing of organic beings, as would be the 

 case through the constant influence of selection. On the contrary, 

 there are entire groups of animals appearing and disappearing. 

 Darwin sought to explain this by means of gaps in our knowledge, 

 but today it is known that these supposed gaps possess too great a 

 horizontal extension. 



There now arises the thought that the changes in the outer con- 

 ditions of life have a controlling influence. I may here state that 

 on this question there was some correspondence between Darwin 

 and our widely mourned Neumayr, and that Darwin in no wise 

 took a dissenting stand against the objections. In this connection 

 it is most remarkable that the great and general knowledge of pale- 

 ontology which I have just indicated, should apparently have made 

 upon so great a mind as Darwin's less of an impression than those 

 small lines of variation noticed in certain fossil fresh-water snails, 

 as, for example, in Valvata or Paludina. 



Here and there conditions are combined which permit somewhat 

 closer analyses of the relations of this subject. This for instance, 

 is the case in the superposition of. the Tertiary land faunas of Europe, 



