Donaldson, American and Europea^t Frogs. 133 



of note that the results obtained on R. pipiens in August, 1904, 

 correspond with the lowest weights found during August, 1901, in 

 the case of the unpubhshed series. 



In one sense this is perhaps fortunate, because it shows that the 

 values here reported for R. pipiens are minimal, and if those for 

 the European forms are also minimal, then the differences are 

 approximately normal. If, on the other hand, the values for the 

 European species are higher than the minimal, then the differ- 

 ences here given are less than they should be. In any case, and 

 this is the main point, it follows that the differences here given are 

 not exaggerated. I conclude therefore that the European species 

 have a central nervous system which weighs from 11 per cent to 

 12 per cent less than that of R. pipiens. 



As the Chart 2 shows, the curves for the weight of the central 

 nervous system run nearly parallel, and as in a previous study 

 (Donaldson '02) R. pipiens has been found to conform to the 

 formula for the determination of this weight, which is based on 

 the body weight and total length of the frog, it follows that the 

 European species would also conform to this same formula. 



The formula contains a constant, C, which is different for 

 each species, and which is modified by the general condition of 

 any series. In the series of R. pipiens of 1902, the value of the 

 constant C. was 28. In the present series of R. pipiens which, 

 as has been noted, yields a low weight for the central nervous 

 system, the value of C. is 26, and we should anticipate that it 

 would be less for the two European species. 



On making the calculations, I find the following values for C. 



R. pipiens C = 26 



R. esculenta C ^ 24 



R. temporaria 0=23 



Our expectation then that the formula for the European species 

 would have smaller values of C. is shown to be warranted. 



On separating the weight of the brain from that of the spinal 

 cord, and recording them separately, we have the relations given 

 in Table ii. 



Presenting these results in a form of a chart (Chart 3) it is seen 

 that the brain weights for the several species follow the same order 

 as that of the weight of the entire central nervous system, the 

 superiority of R. pipiens being even more marked. The weights 

 for the spinal cords however run much closer together. 



