484 ZOOLOGY SINCE DARWIN. 



The conclusion that chromatin is the special vehicle of heredity could 

 not, however, be brought forward with convincing precision until 

 bisexual (digenous) propagation was studied. Here, where two cells, 

 usually of very different shape, unite to form the impregnated egg, it 

 is proved that during the act the chromatin of the latter is formed in 

 equal proportions from the chromatin of the two parent cells. 



So the century-old problem of fecundation has been solved, a prob- 

 lem whose history shows in the most instructive and at the same time 

 amusing manner how preconceived ideas may becloud observation, and 

 how ingenious the human mind is when it tries to replace the want of 

 facts by dialectics. 



The researches on cell division and fecundation, together with the 

 tendency to phytogeny, have given to the post-Darwinian period of zool- 

 ogy its character. They are among the most glorious achievements in 

 the realm of natural science. Those who in the zoological field have 

 specially interested themselves in these investigations — W. Flemming, 

 O. and E. Hertwig, Ed. van Beneden— have for the first time made it 

 possible to formulate a theory of heredity. For, however important 

 may be the knowledge quite recently acquired that all changes in the 

 chromatin are passive, conducted and controlled by a newly discovered 

 source of power, the " centrosome," it can in no way change our con- 

 ception of the chromatin as the substance by which the parental quali- 

 ties are transmitted. 



As all cell nuclei are derived from the nucleus of the egg, every cell 

 of the body must contain a minute portion of the parental chromatin 

 and thereby is secured the transmission of the parental qualities. 

 Upon this basis is founded A. Weismann's theory of heredity, 1 which 

 has certainly performed one service that can not be controverted, in 

 that it has at last clearly formulated the question. 



Are the lines of development exactly predetermined by the constitu- 

 tion of the germ, or is the germ to a certain extent an indifferent 

 constructive material, whose future shaping depends solely upon the 

 conditions of existence to which it is exposed? 



On the one hand, the derivation of existing living forms from ances- 

 tors dissimilar in shape, and, on the other, the phenomena of heredity 

 which teach us that parent and child, or to express it otherwise, the 

 successive generative cycles of the same species, always pass through 

 sijecifically similar morphological states, enable us to find an answer to 

 these questions. It is expressed thus by the orthodox Darwinians: 

 Every organism is a resultant of heredity and adaptation — what the 

 parent inherits he transmits wholly to the child, but he adds thereto 

 what he has himself acquired. 



X A. Weismann : Das Keiuiplasma, erne Theorie der Vererbung, Jena, 1892. Aniong 

 the other numerous writings of this author on the same subject the following are 

 especially worthy of attention: Die Allmacht der Naturziichtung, eine Erwidernng 

 an Herbert Spencer, Jena, 1893; Aussere Einniisse als Entwicklungsreize, Jena, 1894; 

 Neue Gedanken zur Vererbungsfrage, Jena, 1895. 



