590.9 312 



III. 



Zoology since Darwin.' 



Part II. 



ZOOLOGY profited much by the countless researches which con- 

 sciously or unconsciously led towards the solution of the problem 

 of heredity and variation. 



Darwin accepted heredity as a fact proved by the specific re- 

 semblance between parent and child, without seeking after its material 

 carriers. - Since all living beings consist of cells, and even the most 

 complicated organism has in the first beginnings of its development the 

 form of a simple cell, these carriers could only be portions of cells. 

 To find them was left to a deeper insight into the build of the cell and 

 into the meaning of its parts. As the structure of the cell-substance 

 and the nucleus contained therein was investigated with new methods, 

 the surprising fact was discovered that movement and irritability as 

 well as respiration were functions exercised by the cell-substance 

 independently of the nucleus, and that on the other hand assimilation 

 and secretion followed only under the influence of the nucleus, and 

 that this was the only organising form-building factor of the cell. 

 Further research showed the nucleus to consist of two substances, 

 chromatin and achromatin, easily distinguished by their re-action to 

 colouring agents ; the first of these plays an important role in cell- 

 division, since its changes introduce the process of multiplication, 

 and lead, through regularly successive typical forms (mitosis), to 

 division first of the nucleus, and then of the extra-nuclear cell- 

 substance. 



The conclusion that chromatin was the true heredity-substance 

 could only be deduced with convincing clearness after duo-parental 

 (amphigonic) reproduction had been studied. Here, where two cells, 

 usually of different form, combine to form an egg capable of develop- 



1 Lecture delivered by Professor Ludvvig von Graff on his installation as Rector 

 Magnificus of the K. K. Karl-Franzens University in Graz, November 4, 1895. 

 The profits from the sale of the original go to the Freitisch-Stiftung of the 

 University. [Continued from p. 198.) 



2 Darwin's Pangenesis hypothesis, which assumed the germ to be influenced by 

 the body in which it was buried, as well as by the prevailing external conditions, in 

 such a way that minute particles were continually added to it from all parts of the 

 body, has no actual foundation, and is, as Weismann says in his " Keimplasma," p. 7, 

 " more a statement, than an explanation, of the problem of heredity." 



