448 



SCIENTIFIC THOUGHT. 



New 

 problems. 



and centring in van Beneden's discovery, 1 been replaced 

 by definite conceptions capable of typical description. 

 This typical process consists in the fusion of certain 

 parts of the male and female cells, the nuclei or 

 kernels playing an important if not the essential part. 

 Many biologists of the foremost rank, notably in 

 Germany and France, have contributed to make clearer 

 the various lines in this typical picture of the most 

 mysterious process in the physical organism, whilst 

 every new discovery has brought with it new and 

 unanswered questions or given a novel aspect to older 

 problems. 



Of these problems, those of heredity and variation 

 are a present by far the most important. Both 

 the cellular theory of living matter and the theory 

 of natural selection, including the principles of 

 differentiation and of the division of physiological 

 labour, converge upon these two great facts of 

 modern biology. The theory of natural selection pre- 



1 See last note. "Since 

 researches of O. Hertwig 



the 

 and 



others in 1875, it had been clear 

 that each parent contributes a 

 single germ -cell to the formation 

 of the offspring ; but the masterly 

 researches of E. van Beneden 

 (1883) showed that every nucleus 

 of the offspring may contain nuc- 

 lear substance derived from each 

 of the parents, a conclusion which 

 is visibly demonstrable for a few 

 of the first steps in cleavage. In 

 fact, van Beneden to some extent 

 proved what Huxley had foreseen 

 when he said, in 1878, 'It is 

 conceivable, and indeed probable, 

 that every part of the adult 

 contains molecules, derived both 

 from the male and from the 



female parent ; and that, regarded 

 as a mass of molecules, the entire 

 organism may be compared to a 

 web, of which the warp is derived 

 from the female, and the woof 

 from the male ' " ( J. Arth. Thom- 

 son, 'The Science of Life,' p. 

 129). Another theoretical antic- 

 ipation is, according to Haecker 

 (loc. cit., p. 133), the " Idioplasma " 

 of Niigeli : '' The heritable sub- 

 stance, organised, possessing a com- 

 plex structure, transmitted from 

 one generation to another," which 

 was "about the same time identi- 

 fied by Strassburger, O. Hertwig, 

 von Kolliker, and Weismann, with 

 the chromatin substance of the 

 nucleus/' 



