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SCIENCE 



[N. S. Vol. XXXVII. No. 961 



ered here to express our reverence for the 

 man and our admiration for the scholar. 

 It is our part to keep alive the tradition of 

 truth-loving, of scientific devotion and of 

 perfect modesty which is our legacy from 

 Joseph Leidy. Chakles S. Minot 



EEBEDIT7 AND MICROSCOPICAL 

 SESEASCE ' 



I have been much honored by the invi- 

 tation to deliver the first lecture of a series 

 established in honor of Joseph Leidy, a 

 man distinguished alike for the diversity 

 and importance of his original contribu- 

 tions to knowledge and for the far-reaching 

 influence that he exerted on other men of 

 science, in his own time and after. No 

 American naturalist could be named whose 

 biological interests ranged over a wider 

 field; and the selection of my topic this 

 evening has been influenced in some meas- 

 ure by the fact that Leidy was an almost 

 solitary pioneer of microscopical investiga- 

 tion in this country, at a time when the 

 cell-theory was in the earliest stages of its 

 development, and when no one could have 

 imagined the brilliant future that lay be- 

 fore it. Did time permit I would gladly 

 dwell for a moment on his early observa- 

 tions on the structure and division of cells, 

 and on the activities of the simplest forms 

 of life. Much of his subsequent work lay 

 in a very different field of inquiry, but 

 his interest in microscopical investigation 

 never deserted him, witness to which was 

 borne by the publication in his later life of 

 a beautiful monograph on the fresh-water 

 rhizopods, which at once took its place as 

 one of the classics of American zoology. 



^A lecture delivered befere the University of 

 Pennsylvania, AprU 17, 1913, on the Joseph Leidy 

 Foundation. With the exception of three general 

 diagrams it has been impracticable to reproduce 

 the figures that were shown by means of lantern 

 slides. 



More than half a century has passed 

 since Leidy 's earliest studies with the 

 microscope. The main motive power be- 

 hind the unparalleled advance in biology 

 during this period has been the persistent 

 effort to explain the activities of living 

 things through investigations upon their 

 structure, whether anatomical, physical or 

 chemical. This effort entered upon a new 

 era with the discovery of protoplasm and 

 the promulgation of the cell-theory ; for its 

 final objective was now seen to lie in 

 minute structural elements, the cells, of 

 which the tissues are composed. Little by 

 little it became clear that the cell, what- 

 ever else it be, is a microscopic chemical 

 engine, where the energy of the foodstuffs 

 is finally set free, and applied to the 

 work of life. The question inevitably arose 

 whether we can discover within the cell 

 any visible apparatus by which this is ac- 

 complished. The inquiry has a thousand 

 aspects ; I ask your attention to that which 

 relates to the problem of heredity. 



It long since became clear that the ceU- 

 theory offers us a general explanation of 

 heredity. Heredity is a consequence of the 

 genetic continuity of cells by division, and 

 the germ-cells form the vehicle of trans- 

 mission from one generation to another. 

 This fundamental discovery divested hered- 

 ity of the mystery in which it had so long 

 been enveloped, though it must always re- 

 main among the most wonderful of phe- 

 nomena. But this result only cleared the 

 way for further advances. Our scientific 

 curiosity is aroused in the highest degree 

 by more specific problems of heredity. 

 Why do individuals now and then appear 

 that show little resemblance to their im- 

 mediate progenitors but "revert" to much 

 more remote ancestors? Why do the 

 grandparents often exert definite effects 

 upon their grandchildren of which no sug- 

 gestion is given by their children? Wbat 



