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SCIENCE 



[N. S. Vol. XXV. No. 63(> 



assumes that all eggs and every sperm 

 carry the potentialities of both sexes ; there 

 are no male and no female eggs, no male or 

 female sperm, in the sense that each carries 

 only one set of characters, but every germ 

 cell is a sex-hybrid, and the fertilized egg 

 is a double-barreled sex-hybrid. This view 

 maintains that the sex of the embryo is de- 

 termined by an internal condition that is 

 present in the egg or sperm, which leads to 

 the domination of one of the two possible 

 alternatives. This is modern epigenesis 

 as I understand it; predetermination, per- 

 haps, but not preformation through the 

 separation of contrasted characters. From 

 this point of view we can imagine that sex- 

 determination may be reached in more than 

 one way. It may be due to conditions that 

 are present in the sperm or in the egg, or 

 as a result of the union of egg and sperm, 

 for any internal, or even external, condi- 

 tion, that turns the balance one way or the 

 other is a sex determinant. 



It now appears probable that the prob- 

 lem of sex determination is to be sought in 

 the same mechanism that accounts for al- 

 ternative inheritance in general, Mendelian 

 or otherwise. Strasburger, Bateson and 

 Castle have pointed out the close parallel 

 that seems to exist between the two cases. 

 In Mendelian inheritance also we have to 

 face the alternatives of preformation and 

 epigenesis. The currently accepted inter- 

 pretation of Mendelian inheritance is strict- 

 ly one of preformation. Alternative char- 

 acters are treated as entities in the germ 

 cells that may be shuffled, but seldom get 

 mixed. With each new deal the characters 

 are separated, one germ cell getting one 

 character and another the contrasted char- 

 acter. 



If we take the opposite point of view, 

 that of epigenetic development, the out- 

 come, wherever alternative or contrasted 

 characters are involved, is not due to sepa- 

 ration, but to alternative dominance and 



recession, which need not give three types, 

 but only two, if selective fertilization oc- 

 curs, or, if only the egg or the sperm con- 

 tains the internal factor that determines, 

 that one or the other of the alternative sex 

 characters shall predominate. 



Which of these general points of view, 

 preformation or epigenesis, we may think 

 more profitable as a working hypothesis is, 

 I believe, the question of the hour. My 

 own preference — or prejudice, perhaps— is 

 for the epigenetic interpretation, but the 

 whole truth may lie somewhere between 

 these two forms of thought that are the 

 Scylla and Charybdis of biological specula- 

 tion. Thomas Hunt Morgan 



Columbia Univeksitt 



SCIENTIFIC BOOKS 



Ahhandlungen ueber theoretische Physik. 



Von H. A. LoRENTZ. Leipsic, B. G. Teub- 



ner. 1906. 



This work of Professor Lorentz is the result 

 of a request of the publishers asking that a 

 new edition of his articles on theoretical phys- 

 ics be prepared. The first part of the first 

 volume appeared in 1906, and the complete 

 work will consist of two volumes. The titles 

 of the chapters in the first part of the first 

 volume will be given below, together with 

 some remarks indicating the nature and scope 

 of the work. 



Chapter I. — Some Considerations on the 

 Principles of Dynamics in connection with 

 Hertz's ' Prinzipien der Mechanik.' Starting, 

 with the principle upon which Hertz causes 

 the whole science of dynamics to rest, viz., 

 that a material system moves with constant 

 velocity in a path of least curvature, Professor 

 Lorentz investigates the question as to how 

 far the method of Hertz is advantageous if 

 one disregards Hertz's hypotheses concerning 

 hidden motions and considers the system under 

 the action of forces in the usual sense of the 

 word. 



After presenting in his own clear and con- 

 cise way the fundamental notions of Hertz, 

 the author shows how they lead to the general 



