544 THE HISTORY OF BIOLOGY 



every animal has an equal number of chromosomes in each cell. In connexion 

 therewith he discovered the reduction in the number of chromosomes in the 

 sexual cells: that upon the latter's maturation-division the number of chro- 

 mosomes in both the male and the female elements are reduced to half of 

 the normal number, which is again restored upon fertilization, when the 

 male and female chromosomes are united. Somewhat later Karl Rabl (1853- 

 1917), professor at Leipzig, detected the individuality of chromosomes: that 

 in a cell every chromosome originates from a given chromosome, like itself 

 in form and size, in the mother cell. And finally, in 1901, the American 

 W. S. Sutton discovered the so-called accessory chromosome, which at the 

 nuclear division assumes a place for itself. All these facts have played a de- 

 cisive part in modern heredity-research and will be further developed later 

 on in this work. 



As a result of these investigations into fertilization,^ very briefly re- 

 ferred to above, our knowledge of the phenomena of life was so considerably 

 enhanced that it is difficult to overestimate its value; this not least because 

 the same fertilization-phenomena were established in the vegetable at the 

 same time as in the animal kingdom: the union of male and female nucleus, 

 the reduction of the chromatin, and the individuality of the chromosomes; 

 all these processes take place with a certain number of modifications, but 

 on the same principle in every multicellular organism. Life has thereby been 

 given a uniformity far more demonstrable and real than the hypothetical 

 common descent of Darwinism. Even in the lowest unicellular organisms, 

 whether they belong to the animal or the vegetable kingdom, a similarity 

 in their evolution has been definitely established. We shall now proceed to 

 discuss these forms. 



3 . Microbiology 



The Darwinists of the earlier school, chiefly Haeckel, largely interested 

 themselves, as we have seen, in the very lowest animal forms; it was expected 

 that they would produce fresh ideas in regard to the origin of life upon the 

 earth, discoveries that would fill the gap between living and lifeless sub- 

 stance and would thus make the great evolutional series in the universe 

 entirely uniform. These expectations, however, whether associated with 

 Huxley's bathybius slime or with Haeckel's Monera, have not been fulfilled; 

 bathybius turned out to be a lifeless calcareous deposit, and in the Monera 

 have been found nuclei and other organic details giving evidence of ordinary 



^ In an article entitled " Dokumente xur Geschkhte der Zeugungshhre" (in Archiv fur micro- 

 scopische Anatomic, Bd. 90), O. Hertwig has given a comprehensive account of the history of 

 fertilization-research up to the year 1917. 



