14 



HENRY H. DONALDSON 



As is to be seen by inspection of the foregoing table 11 the 

 value of C for the 1904 records is greater in both the European 

 species than for the corresponding 1909 records, and as noted 

 above, the greatest difference (1.6) is in the case of R. esculenta. 



In connection with this table a word of explanation is required. 

 It has been found that there is a slight increase in the value of C 

 as the absolute size of the frog increases. This is a relation pre- 

 viously overlooked, but which will be discussed elsewhere. The 

 bearing of it on the present case is that in making a comparison 

 of the values of C in any pair of records, it is necessary in order 

 to get trustworthy results, to compare the determinations for 

 frogs of approximately the same range in size. In the present 

 instance this makes it necessary in the case of R. esculenta to 

 omit the value of C for the first weight group of the 1904 series, 

 because there is no corresponding weight group on the 1909 series, 

 and similarly to omit the determinations for the last weight group 

 of the 1909 series. 



A glance at chart 2 will serve to supplement the explanation. 



In the case of the records for R. temporaria, the values for C in 

 all the weight groups of both years have been used in making up the 

 averages. It is because of this influence of the absolute size that 

 the average body weights for each series are entered in table 11. 



All through the present paper the data on R. pipiens used in 

 1904 have been repeated without revision. In the former com- 

 munication (Donaldson '08. pp. 132-133) it was noted that the 

 weight of the central nervous system in the series of this species 

 was low in comparison with other data which we had. This 

 statement still holds good, but it was thought wiser to leave the 

 standard as represented by 1904 records on R. pipiens unchanged 

 at this time. 



As evidence that the weights here used were low for this species, 

 I give below two other series of determinations of C on Chicago 

 frogs as follows : 



