184 DARWINISM TO-DAY. 



suppose that the long-tailed individuals are preserved by this extra 

 couple of inches at the expense of the shorter-tailed individuals 

 in order that the character may be within the reach of natural 

 selection. If the animals are troubled by insect pests, it is cer- 

 tainly a matter of convenience to them to have a tail long enough 

 to brush off the flies, and the longer tail, within certain limits, will 

 be more useful than a shorter one. It is not likely that this will 

 preserve the life of a single individual, but it will follow that the 

 animals with longer tails will be less irritated by insects than 

 those with shorter tails. Now, although this would not affect the 

 matter of life and death, a nervous irritation would pretty surely 

 interfere with the reproductive efficiency. An animal that is con- 

 stantly bothered by insects will have less nervous energy to devote 

 to reproduction, and. therefore, such a constantly irritated animal 

 would be likely to be somewhat less prolific than one less irritated. 

 From this it would follow that the half of the animals with tails a 

 little longer than the average, would be pretty sure to leave a some- 

 what larger number of offspring than .the half whose tails were 

 below the average. But a slightly increased fertility of this sort 

 would, in the course of a few generations, see the long-tailed animals 

 becoming more and more numerous, until they w r ould eventually 

 replace the others." 



3 Dodel-Port, A., "Wesen und Begriindung der Abstammungs- 

 und Zuchtwahl-Theorie in zwei gemeinverstandlichen Vortragen," 



1877- 



1 The tsetse-fly (Glossina sp.~), long notorious as a terrible 

 pest of cattle in Africa, produces its ravages by disseminating 

 (through biting, /. c., puncturing the skin) the specific causes 

 (certain minute blood-inhabiting parasitic one-celled animals 

 known as trypanosomes) of the plague called Nagana (fly dis- 

 ease). 



5 For an elaborate discussion of the principle of correlation (not 

 bearing perhaps except in a general way on the point just at issue. 



References to but of much general interest) see Radl, Em., "Uber 

 papers on corre- die Bedeutung des Prinzips von der Korrelation in 

 lation. der Biologic," Biol. CcntralbL, Vol. XXI. pp. 401- 



416, 490-496, 550-560, 605-621, 1901. See also Webber, H. J., "Cor- 

 relation of Characters in Plant-Breeding," Proc. Amer. Breeders' 

 Assoc., Vol. II, pp. 73-83, 1906. 



6 For examples see R. Meldola, "The Utility of Specific Char- 

 acters and Physiological Correlation," Proc. Ent. Soc., London, pp. 

 62-92, 1896; also A. R. Wallace, "The Problem of Utility; are 

 specific characters always or generally useful?" Jour. Linn. Soc., 

 Vol. XXV, pp. 481-496, 1894. 



