142 ANIMAL AGGREGATIONS 



Under conditions of high toxicity the animals, or, in the case of 

 Child's worms, certain regions, which have the lowest rate of general 

 metabolism are least affected by the toxic agent and survive longest. 

 With weaker solutions, on the other hand, the most vigorous indi- 

 viduals, or, in the case of the worms, the most vigorous regions, can 

 acclimate most readily and hence survive longer. Fowler's results 

 show that depression due to crowding may have definite survival 

 value under certain conditions but that with weaker concentrations 

 of even the same salts crowding decreases the chance of survival. In 

 the latter aspect his results support the facts reported by Drzewina 

 and Bohn in their experiments with KCl. Fowler's results fail to 

 support the hypothesis that a specific autodestructive material is 

 produced. They extend the later explanation of Drzewina and Bohn 

 by indicating that the unknown autodestructive substance is the 

 carbon dioxide produced by the animals, which does raise the H-ion 

 concentration as Drzewina and Bohn determined. 



For further consideration of the relation between density of popu- 

 lation and the death-rate it seems best to take up the case with re- 

 spect to man, since with human populations this relationship has 

 attracted particular attention for a considerable period of time. We 

 have already referred to the generalization known as Farr's law; this 

 states that if the death-rate be represented by R and the density of 

 population per unit area by D, then R = cD"\ where c and m are con- 

 stants. 



Brownlee (191 5) rehabilitated this law by showing that the statis- 

 tics used by Farr, which came from the decade 1861-70, compared 

 favorably, so far as the relation between population density and 

 death-rate was concerned, with those of the decade 1891-1900, as 

 given by Tatham. Brownlee's republication of these tables and his 

 calculations are given herewith; see Table IV. 



Brownlee calls attention to the fact that the values of m corre- 

 spond roughly for each type of analysis in the two periods but that 

 in the case of the life-table death-rates they correspond to the third 

 decimal place, which is as much as could be statistically expected. 

 He concludes that Farr's law is thus shown to be a definite law oper- 

 ating independently of the changes due to sanitary progress. Re- 



