CHROMOSOMES AND MENDELIAN HEREDITY 285 



generation. Even wholly new variations permanently affecting the 

 germinal constitution and thus modifying the characters of subsequent 

 generations are supplementary, rather than opposed, to heredity. In 

 the general process of heredity we witness the results of processes occur- 

 ring in a protoplasmic system which, though it maintains a specific type of 

 organization through successive generations, undergoes minor alterations 

 which modify the characteristics of the race. Hence it may be said that 

 heredity is the occurrence of related hut not necessarily identical conditions, 

 events, or characters in successive generations of organisms as a consequence 

 of their protoplasmic organization. 



From these considerations it follows that the problem of heredity 

 which confronts the cytologist is that of ascertaining in what respect 

 resemblances and differences in the characters manifested by successive 

 generations are correlated with similarities and dissimilarities in the 

 organization of the protoplasts with which the successive developmental 

 cycles begin, and how such constitutional conditions arise. Search is 

 to be made for a mechanism which remains comparatively stable as it 

 operates through regularly recurring ontogenetic cycles, and yet under- 

 goes orderly alterations of a kind which will help to account for the 

 observed phenomena of heredity and variation, as well as to approach 

 an understanding of evolutionary advance. 



The method followed in such studies is mainly that of altering the 

 cytological constitution of the organism and observing the effect of this 

 upon the inherited characters. Such alterations may be induced by 

 various means, but the one which concerns us in this chapter is that of 

 crossing individuals unlike with respect to cytological or external charac- 

 ters and noting the results in following generations. In this way valuable 

 evidence is gained regarding the cytological basis of the inheritance of 

 those characters in which the individuals may differ. 



The Role of the Nucleus. — The hypothesis that "the nucleus of 

 the cell is the principal organ of inheritance" was suggested by Haeckel 

 in 1866. Cytological evidence in support of this view was brought 

 forward by a number of workers who described the behavior of the nucleus 

 in the various stages of the life cycle, particularly in somatic cell-division, 

 syngamy, and meiosis (p. 441). Of special interest was the discovery by 

 0. Hertwig and Strasburger that in animals and plants the two nuclei 

 which fuse in the process of syngamy are derived from the two gametes 

 and hence from the two parents. Since it was only in their nuclei that 

 the gametes appeared structurally alike in higher organisms, and since 

 inheritance from the two parents in general seemed to be equivalent, 

 Haeckel's hypothesis was advanced to the rank of a theory, and it was 

 not long before the nucleus was assigned a monopoly in hereditary 

 transmission. 



