LINKAGE 397 



which it brings together under one point of view a multitude of facts that other- 

 wise remain disconnected and unintelligible. What arrests the attention when 

 the facts are broadly viewed is the unmistakable parallel between the course of 

 heredity and the history of the chromatin-substance in the whole cycle of its 

 transformation. In respect to some of the most important phenomena of 

 heredity it is only in the chromatin that such a parallel can be accurately traced. 

 It is this substance, in the form of chromosomes, that shows the association of 

 exactly equivalent maternal and paternal elements in the fertilization of the 

 egg. In it alone do we clearly see the equal distribution of these elements to 

 every part of the body of the offspring. In the perverted forms of development 

 that result from double fertilization of the egg and the like it is only in the 

 abnormal distribution of the chromatin-substance by multipolar division that we 

 see a physical counterpart of the derangement of development. Only in the 

 chromatin-substance, again, do we see in the course of the maturation of the 

 germ cells a redistribution of elements that shows a parallel to the astonishing 

 disjunction and redistribution of the factors of heredity that are displayed in the 

 Mendelian phenomenon." 



With more particular reference to the chiasmatype hypothesis Wilson 

 (1913) says: 



"This, admittedly, is a bold venture into a highly hypothetical region. Its 

 justification is the pragmatic one that it 'works.' The hypothesis gives us the 

 only intelligible explanation that has yet been offered for a series of undoubted 

 facts ; and it is certainly worthy of the most attentive further examination . . . 

 We have much to gain and nothing to lose by the use of explanatory hypotheses 

 that are naturally suggested by the facts and help us to formulate them for 

 analysis, so long as such hypotheses are not allowed to degenerate into dogmas 

 accepted as an act of faith, but are only used as instruments for further observa- 

 tion and experiment." 



Bibliography at end of Chapter XVIII. 



