Operation of the Porous Cup Atmometer. Ill 



of experimental science. The result has been the accretion of a 

 body of evidence showing that discontinuous variation leading 

 to the possible production of new forms is not confined to the 

 evening primroses, and while it seems more common among 

 plants, evidence from animals is not lacking. 



Not the least interesting result of the whole movement has 

 been the awakening of a vast chorus of voices crying the vir- 

 tues of special interests and controverted theories. Those busy 

 with vitalism of various types, have spun a moiety of the tenuous 

 web of their favorite fabric. Isolation and geographical dis- 

 tribution have again been advanced as the prime factors in 

 species making. Natural selection with diverse meanings has 

 been called upon to account for all evolutionary development. 

 Orthogenesis has once more had its creed re-stated, and La- 

 raarckianism, with its predication of the inheritance of ac- 

 quired characters has again come into the limelight, with the 

 result that its claims seem about to be tested experimentally 

 as has not been done before. 



It is as if all these claimants had been called before the bar 

 of public opinion for a re-hearing as to the validity of their 

 theses. It would be singular if the standing of some of the ideas 

 represented were not altered in such comprehensive review and 

 revaluation. 



The appearance of the translation of the mutation theory 

 with its original records of experimentation, will make the techni- 

 cal evidence upon which its generalizations are based, accessible 

 to a great constituency of workers and will do much to remove 

 prevalent misapprehensions concerning some of the principal 

 features of the theory. 



OPERATION OF THE POROUS CUP ATMOMETER. 

 By Burton Edward Livingston. 

 The porous cup atmometer, for measuring evaporation, was 

 first devised in its essentials, by Babinet in 1848 (Compt. Rend., 

 27:529-30), but attracted no particular attention. * 



It was independently devised by Mitscherlich (Landw. 

 Versuchs. Stat., 60: 63, and 61 : 320, 1904) and also by the author, 



♦The literature of evaporation is brought together in an anaotated bibliography of 

 the subject published by Grace J. Livingstoa in the Monthly Weather Review. 1908-09. 



