1883.] NATURAL SCIENCES OF PHILADELPHIA. 2T3 



1. Accepting the data as given, are tlie conclusions drawn from 

 them necessarily correct ? 



2. Are the data themselves correct ? 



The first of the questions is answered by a negative in interro- 

 gation, if so it may be termed. Prof. Gill objects to my (?) method 

 of distinguishing between the larger and smaller zoogeographical 

 divisions, and pointingly submits that "The question may natur- 

 ally recur, why the line which separates 'regions' from 'sub- 

 regions ' should be drawn between 35 and 46 per cent, rather than 

 between 46 and 63 or 64 per cent., or even between 64 and 78 per 

 cent. Prof. Heilprin has not told us why, and I am unable to appre- 

 ciate the reason therefor. Surely it is not sufficient to answer by 

 simply asking the question put in Nature (p. 606)." The problem 

 here stated is certainl}^ one that does not admit of a ready logical 

 solution, and one which the writer has never attempted to solve ; 

 nor, as far as he is aware, has its solution ever been effected by 

 an}' other writer on zoogeography. 78 is indisputably as near to 

 64 as this last is to 46, and but little less near than 46 is to 35 ; 

 and if one or two more terms be added to the series, it may still 

 be contended with equal justice that 46 holds approximately the 

 same relation (in this sense) to 35 as 35 does to 25, and 25 to 15 

 as 15 to 5, and so to either end. So far, well and good. But the 

 fact still remains, nevertheless, that a region whose fauna is char- 

 acterized by 90 or 78 per cent, of peculiarities is eminently well 

 defined from any and all other regions; that one whose peculiar- 

 ities amount to 64 or 46 per cent, is considerably less well-defined; 

 and that another, where the peculiarity amounts to only 15 or 10 

 per cent., is still less well-defined, and, in fact, scarcely defined at 

 all. If a line of division or separation is to be drawn at all it 

 must be drawn somewhere, and this somewhere must be dictated 

 in great part b}' common sense. 



As regards the second question (2), Prof. Gill is much more 

 emphatic in his (negative) repl}'. In the first place, it is pleaded 

 that the marine mammals ought to have been excluded from any 

 analysis bearing upon the subject of zoogeography, "because 

 their distribution and limitation are determined by other factors 

 than those which regulate the terrestrial ones." But surely if 

 these forms are to be excluded, we might for almost identical 

 reasons exclude the birds, since in the distribution of this class 

 of animals factors are involved which are in no way operative in 



