396 INTRODUCTION TO CYTOLOGY 



proportion of cases without involving a rupture of the chromosomes. These 

 forces, however, are purely conjectural. It is pointed out by Jennings 

 (1918), moreover, that the gametic ratios theoretically resulting from 

 such a process do not agree with the actual ratios observed in Drosophila. 



Value of the Chromosome Theory of Heredity. Whatever judgment 

 may ultimately be rendered on the chromosome theory of heredity as 

 outlined in these chapters, it must be agreed that the value of this theory 

 in the present state of our knowledge can hardly be overestimated. 

 Through its use a huge number of the observed facts of inheritance are 

 being reduced to order: the painstaking investigation of the interrelation- 

 ships of all the known heritable characters of even a single organism 

 such as Drosophila cannot fail to be a great service to biological science. 

 Its appeal to the cytologist, as Wilson states, is largely through the man- 

 ner in which it seeks to make use of known cell mechanisms rather than 

 entirely hypothetical processes. Those portions of the theory which 

 are as yet unsupported by the results of direct cytological observation, 

 though not contradicted thereby, at least have the virtue of affording a 

 useful and graphic representation of the mutual behavior of hereditary 

 characters. Notwithstanding the statement that " the graphic repre- 

 sentation of the location of the factors is a type of representation common 

 to every set of phenomena which can be expressed as percentages" (Trow 

 1916), these hypotheses are of great value, for by aiding in the correla- 

 tion of the facts of inheritance they serve to increase the number of 

 observed phenomena statable in terms of order; and the reduction of 

 experience to order and the statement of this order in simple formulae, 

 together with the search for new truth, constitute the principal tasks 

 of science. If this work of correlation has been well done the whole 

 body of facts can readily be placed under another theoretical interpre- 

 tation and described in a new set of terms should occasion require. 



Although it may be that the chiasmatype hypothesis of linkage is in 

 certain points inadequate, mathematically (Trow 1916) or otherwise, it is 

 nevertheless true, as we have already seen, that it fits the case very well. 

 At the same time we may remind ourselves that the fact that a hypothesis 

 works well is no guarantee of its ultimate truth. But even if the chiasma- 

 type interpretation should have to be ever so greatly modified as new 

 facts accumulate, it is scarcely to be doubted that the chromosome theory 

 of heredity in some form will turn out to be in accord with the truth. 

 With respect to this general theory Wilson (1909) writes as follows: 



"I stand with those who have followed Oscar Hertwig and Strasburger in 

 assigning a special significance to the nucleus in heredity, and who have recog- 

 nized in the chromatin a substance that may in a certain sense be regarded as the 

 idioplasm. This view is based upon no single or demonstrative proof. It 

 rests upon circumstantial and cumulative evidence, derived from many sources. 

 The irresistible appeal which it makes to the mind results from the manner in 



