754 



SCIENCE 



[N. S. Vol. XXXVI. No. 935 



the rate of survival changes most rapidly with 

 the shorter antennse. 



Dr. Eobert W. Hall and I have submitted 

 Palcemonetes vulgaris to changed salinity. 

 Figs. 11 and 12 show that where the marine 



Fig. 11. Survival rate in Jolmson and Hall 

 experiment in exposing shrimp from marine sal- 

 inity to fresh water. First line of numbers, num- 

 ber of rostral spines; second line, survival rate. 



shrimps were placed in fresh water, the sur- 

 vival curve is higher for those of few rostral 



some of these brackish water shrimp were sub- 

 jected to brine the survival curve was higher 

 for the larger number of spines ; that is, higher 



fSl Im m 5x1 



jtn no j« lis. 



Fig. 12. Survival rate in Johnson and Hall 



experiment in exposing shrimp from brackish 



water to brine. First line of numbers, number of 



rostral spines; second line, survival rate. 



for the marine conditions. The difference in 

 the former case not being well marked, the ex- 



FiG. 13. 



Various theoretical survival curves (dotted) plotted 

 upon a theoretical polygon of frequency. 



spines. Few spines is the normal condition 

 in the fresh water species. The brackish-water 

 strain of the marine species has also fewer 

 spines than the typical marine species. When 



periments should be repeated on a larger scale 

 and with greater refinements, before drawing 

 definite conclusions. In the latter case, how- 

 ever, the result clearly shows selection. 



