FERTILISATION 199 



be aggregated together to comprise units of a still higher order, 

 known as ids. These are identified with the chromatin granules. 

 Every part of the organism (or every character that it possesses) is 

 believed to be represented in an id. Moreover, Weismann assumes 

 that the ids vary slightly in related . individuals, the differences in 

 the ids corresponding with the variations in the species. Lastly, the 

 ids are said to be arrayed in linear series so as to form idants. 

 Weismann identifies these with the chromosomes. It follows, 

 therefore, that each chromosome represents a particular group of 

 slightly differing germ-plasms. The purpose of variation, as 

 expressed in the terms of this theory, is to produce new combina- 

 tions of heritable variations by the mixture of different ids. And 

 since the number of chromosomes, and consequently the number of 

 ids, is doubled as a result of the conjugating process, the complexity 

 of the chromatin would become indefinitely increased if there were 

 no periodic reduction. But this, according to Weismann,. is provided 

 against in the maturation process of:. the gametes, when the quantity 

 of chromatin in the cells becomes reduced by one-half, as described 

 in the preceding chapters. 



The reduced number of chromosomes is supposed to contain all 

 the primary constituents of each of the two parents. And what is 

 more, according to this theory, every gamete contains ids which are 

 derived, not only from both the parents, but also from the ancestors, 

 all the immediate ancestors being represented. 



Weismann's theory of the nature of fertilisation was accepted by 

 many biologists as a working hypothesis until the disinterment of 

 Mendel's discovery about twenty years ago. The confirmation of this 

 discovery by numerous workers in different fields has led to a 

 revision of many of Weismann's conceptions. 



The original experiments of Mendel^ were upon hybridisation 

 in peas, the two parent varieties initially selected differing from each 

 other in one particular character. The hybrids produced by crossing 

 were all similar superficially, and resembled one of the parents in 

 the character in question, which was therefore called the dominant 

 character, the other character being known as recessive. When the 

 hybrids were crossed among themseh'es, approximately one-half of 



' Mendel, "Versuche iiber Pflanzen Hybriden," Verh. natur. f. Yer., in 

 Briinn, vol. iv., 1865. Eeprinted in English in Mendel's Principles of Heredity 

 (Bateson), Cambridge, 1902. Mendel's ■work was rediscovered and confirmed 

 by de Vries, Correns, and Tchermak in 1900, and subsequently by Bateson 

 and a large number of other workers. For a general account of the Mendelian 

 theory, and numerous references to the literature of the subject, see Bateson, 

 loc. ait. ; also Bateson, Saunders, Punnett, and Hurst, etc-., in Reports to the 

 Evolution Committee of the Royal Society, Parts I., II., III., IV., and V., 

 1902, 1905, 1906, and 1909 ; Punnett, Mendelism, 5th Edition, London, 

 1919 ; and Morgan, The Physical Basis of Heredity, Philadelphia and London, 

 1920. 



