lo University of Michigan 



Such, then, in brief, has been the "method of evolutiou" 

 of the North American Unionidse, both in the animals them- 

 selves, and in the causes which have brought these changes 

 about, and, in conclusion, it may be summarized somewhat as 

 follows : 



1. So far as the animals of the Unionidse are concerned,, 

 the evolution from the most primitive forms that are known, 

 to the most complex that now exist, has been all centered 

 around the adaptation of the gills of the female for the care 

 of eggs until they are hatched. Whatever changes have been 

 brought about in the shell have been a necessary corollary to 

 the changes in the animal itself. 



2. The causes of evolution, aside from the innate tendency 

 to variation that is present in all animal life, must be recog- 

 nized in the history of the original migration brought about 

 by the opportunities afiforded by the earliest drainage sys- 

 tem of the country and the subsequent isolation effected by 

 changes in the earth's surface. All these have combined to 

 affect the animal, and to intensify its innate tendency to varia- 

 tion. 



The evolution of the species is a minor matter. It is al- 

 most wholly the result of local influences and is too compli- 

 cated and, in many details, as yet too uncertain to justify an/ 

 attempt to consider it at the present time. 



