142 C. B. Davenport. 



the shells of the three epochs; the shells from the bluff being, how- 

 ever, more like each other than like those of Morehead. 



The series show that the number of rays in both right and left 

 valves has diminished since the Pliocene and that the reduction 

 had made progress during the interval from the lowest deposits 

 in Jack's Bank to the uppermost deposits. They show also that 

 the change in question has been of the quantitative order rather 

 than ot the qualitative or mutational. 



Again, the shells have been becoming more globose. For, the 

 ratio of transverse diameter of either the right or the left valve to 

 its dorso-ventral diameter has increased. Although the recent 

 P. irradians is twice as globose as the fossil P. eboreus the later 

 fossil deposits show a change from the earlier in the same direction 

 and make it probable that a quantitative change that was in 

 progress in geological times has continued to the present time. 



Finally, both valves have been getting more nearly circular — 

 the antero-posterior diameter becoming more nearly equal to the 

 dorso-ventral one. Here, again, there is a quantitative change in 

 a character; not the introduction of a new one. 



Apart from a certain bleached appearance of the shell and its 

 less weight (both due, in part at least, to postmortem changes) 

 the fossil shells differ from the recent ones, so far as I can see, in 

 no other respects than the three enumerated above. It seems 

 justifiable, therefore, to conclude that the evolution from the one 

 species to the other has been without mutation and solely by 

 graduated variation. 



SUMMARY. 



The process of evolution has taken place by various methods 

 and not always in the same way. It is no more justifiable to 

 maintain that all evolution is by mutation than that evolution 

 has always proceeded by slow stages. The best evidence for 

 slow evolution is found in wide-ranging species which while 

 differing greatly at the limits of their range exhibit all gradations 

 in intermediate localities (Melospiza, Pecten); also in fossil series 

 (Pecten eboreus and P. irradians) where the change from one 

 horizon to the next is of the quantitative order. Thus evolution 

 may take place without mutation. 



Station for Experimental Evolution, 



Cold Spring Harbor, N. Y., 



January 24, 1905. 



