November, 1896. ZOOLOGY SINCE DARWIN. 313 



ment, the conclusion was confirmed that the chromatin of the egg 

 arose in equal parts from the chromatin of both parent-cells. 



Thus was solved the problem of centuries — the problem of 

 fertilisation, whose story shows, in the most instructive and, at the 

 same time, most amusing manner, how preconceived ideas can dim 

 observation, and how ingenious the human mind becomes when it 

 is a question of supplying want of fact by dialectics. 



Next to phylogenetic research, investigations on cell-division 

 and fertilisation stamp the post-Darwinian period of zoology. They 

 are among the most brilliant achievements in the domain of natural 

 science. The zoologists who took chief part in them, W. Flemming, 

 O. and R. Hertwig, Ed. van Beneden, were the first^ to render pos- 

 sible the exposition of a theory of heredity. For, weighty as may be 

 the most modern discovery that all movements of chrom^atin are 

 accomplished passively, guided and directed by a source of energy 

 placed in the newly-found centrosome, it effected no change in the 

 interpretation of the chromatin as the substance that transmits the 

 parental characters. 



Since all cell-nuclei come from the nucleus of the ovum, there is in 

 all body-cells a part of the parental chromatin, and thereby an inherit- 

 ance of the parental characteristics is insured. On this foundation rests- 

 Weismann's theory of heredity,- to which we cannot deny the one merit 

 of having at last clearly formulated the question of inheritance. 



Are the ways of evolution exactly determined beforehand by the 

 constitution of the germ, or is the germ a mass of formative substance, 

 indifferent up to a certain point, and governed in its further develop- 

 ment solely by the conditions to which it is exposed ? 



The descent of modern organisms from ancestors of different form 

 on the one hand, and the facts of heredity, which teach us that 

 parent and child (or, broadly put, the successive generations of the 

 same species) always pass through specifically similar stages of form 

 on the other hand, give the answer to this question. As the orthodox 

 Darwinian expresses it : Every organism is the result of heredity and 

 adaptation ; what the parent inherits it transmits in its entirety to the 

 child, but adds thereto what it has itself acquired. 



Weismann's Neo-Darwinism, in opposition to this, denies the 

 inheritance of acquired characters ; and there exists neither a single 

 undoubted fact to disprove his teaching, nor any theory that can 



1 Note added by Author, October, 1S96: When writing this part of my address, I 

 was fully aware that the name of O. Biitschli should have figured in the front rank 

 of researchers on cell-division and fertilisation. Its omission was due to a lapsus 

 calami which I deeply regret, but which I cannot understand ; indeed, I only became 

 aware of it through reading Biitschli's review of my address in Zool. Centralblatt, 

 iii., p. 421. I am glad that this publication in Natural Science enables me in a 

 measure to repair the omission. 



2 A. Weismann, " Das Keimplasma : Eine Theorie der Vererbung." Jena, 1892. 

 This author has also dealt with the question in " Die Allmacht der Naturziichtung : 

 Eine Erwiderung an Herbert Spencer." Jena. 1893. " Aussere Einfliisse als 

 Entwickelungsreize." Jena, 1894. " Neue Gedanken zur Vererbungsfrage.'" 

 Jena, 1895. 



Z 



