December 1, 1905.] 



SCIENCE. 



699 



less impassable walls of chance within a 

 much more circumscribed area, which we 

 may call the practical limit of migration— 

 that is, a limit beyond which any given 

 percentage of units which we like to select 

 do not generally pass. Lastly I tried to 

 apply this reasoning to the important par- 

 ticular case of the immigration of mos- 

 quitoes into an area in which their propa- 

 gation has been arrested by drainage and 

 other suitable means. My conclusions are : 



1. The mosquito-density will always be 

 reduced, not only within the area of opera- 

 tions, but to a distance equal to the ideal 

 limit of migration beyond it. 



2. On the boundary of operations the 

 mosquito-density should always be reduced 

 to about one half the normal density. 



3. The curve of density will rise rapidly 

 outside the boundary and will fall rapidly 

 inside it. 



4. As immigration into an area of opera- 

 tions must always be at the expense of the 

 mosquito population immediately outside 

 it, the average density of the whole area 

 affected by the operations must be the same 

 as if no immigration at all has taken place. 



5. As a general rule for practical pur- 

 poses, if the area of operations be of any 

 considerable size, immigration will not very 

 materially affect the result. 



In conclusion, it must be repeated that 

 the whole subject of mosciuito-reduction 

 can not be scientifically examined without 

 mathematical analysis. The subject is 

 really a part of the mathematical theory 

 of migration — a theory which, so far as I 

 know, has not yet been discussed. It is 

 not possible to make satisfactory experi- 

 ments on the influx, efflux and varying 

 density of mosquitoes without such an 

 analysis — and one, I may add, far more 

 minute than has been attempted here. The 

 subject has suffered much at the hands of 

 those who have attempted ill-devised ex- 



periments without adequate preliminary 

 consideration, and whose opinions or results 

 have seriously impeded the obviously use- 

 ful and practical sanitary policy referred 

 to. The statement, so frequently made, 

 that local anti-propagation measures must 

 always be useless, owing to immigration 

 from outside, is equivalent to saying that 

 the population of the United States would 

 remain the same, even if the birth rate 

 were to be reduced to zero. In a recent 

 experiment at Mian Mir in India the as- 

 tounding result was obtained that the mos- 

 quito-density was, if anything, increased 

 by the anti-propagation measures— which 

 is equivalent to saying that the population 

 of the United States would be increased by 

 the abolition of the birth rate. It is to be 

 hoped that if such experiments are to be 

 repeated they will be conducted by ob- 

 servers who have considered the subject. 

 In the meantime, I for one must continue 

 to believe the somewhat self-evident theory 

 that anti-propagation measures must al- 

 ways reduce the mosquito density— even if 

 the results at Havana, Ismailia, Klang, 

 Port Swettenham and other places are not 

 accepted as irrefragable experimental proof 

 of it, Ronald Ross. 



The Liverpool School of 

 Tropical Medicine. 



SCIENTIFIC BOOKS. 

 A Text-Booh of Physics: Heat. By J. H. 



PoYNTiNG and J. J. Thomson. London, 



Charles Griffin & Co. 1904. Pp. xvi + 



354. 



This text-book is the third of a series on 

 general physics by the two distinguished 

 scholars of Birmingham University and of 

 Cambridge. The other two volumes are 

 ' Properties of Matter/ which has already 

 reached a second edition, and ' Sound,' the 

 third edition of which has recently appeared. 

 Two more volumes, on ' Light ' and ' On Mag- 

 netism and Electricity,' are in preparation. 

 As Professor Poynting says in the preface to 



