2 University of Michigan 



upon the original innate tendency toward variation through 

 changes in environmental conditions resulting from migration, 

 isolation, changes in climate, food supply, and chemical con- 

 stituents in the water, have, first stimulated and then by the 

 pruning off, through natural selection, of improvident varia- 

 tions, guided and directed, as it were, the organism, not only 

 to its present, greater complexity, but to its more perfect 

 adaptation to its present surroundings. Whether the original 

 primitive type, if there had been no changes in environmental 

 conditions, but simply through competition and the struggle 

 for existence, would have evolved along the same lines that it 

 has under the changes of environment to which it has actually 

 been subjected in the past ages, is at the best only a matter of 

 speculation. But there can be no doubt but that the more 

 complex organization of the recent fauna is the combined re- 

 sult of the innate tendency to variation and the influence of 

 changes in the external environmental conditions. The two 

 are coincident. 



For this reason, therefore, taking the North American 

 Unionidse as a concrete example, I propose to discuss briefly 

 both aspects of the question. 



I. CiiAXGEs IN THE Animal. 



The Unionidae are a very ancient family. While it is prob- 

 able their precursors are to be found in the bivalves, which, 

 both in this country and in Europe, have been discovered in the 

 Coal Measures, the connecting links are too few, and the in- 

 terval of time too great to make that a matter of profitable 

 speculation. The first genuine Unios now known in this coun- 

 try have been found in the Triassic of Texas. They are all 

 simple forms and not unlike many of the simpler forms found 

 in the recent fauna. In the Cretaceous deposits in the western 



