Nature of Heredity and Problem of Mechanism 3^5 



be direct evidence for the chromosoiiial hypothesis of 

 heredity. 



And thus it comes to pass that evidence on the problem 

 of the mechanism of heredity will be of two sorts, direct and 

 indirect. Direct evidence will be that obtained by immediate 

 observation of the actual transformation of substances and 

 parts into other substances and parts known to be heredi- 

 tary. The methods here will be those of ordinary histo- 

 genesis and organogenesis, only carried on witii reference 

 to the hereditariness of the parts produced. Indirect evi- 

 dence, on the other hand, will be any sort of evidence which 

 does not come from immediate observation as above indi- 

 cated, but depends upon some measure of inference inter- 

 posed between the observation and the conclusion. By far 

 the greater part of the evidence of this kind being used at 

 present in researches on heredity comes from observations 

 on the germ-cell stages at one end and on adult stages at the 

 other end, of the ontogenetic series. From these observa- 

 tions the inference is drawn by various courses of reasoning 

 that the obsei'ved adult structures and attributes are de- 

 pendent upon and explained by the observed germ-cell sti-uc- 

 tures and attributes. 



Our next task will be that of examining both these sorts 

 of evidence with special reference to the problem of heredi- 

 tary substance as it presents itself in present-day genetic 

 research, namely that of whether nuclear substances, espe- 

 cially chromatin, or cytoplasmic substances, are the 

 "mechanism of heredity." 



REFERENCE INDEX 



1. Conklin ('15) 103 8. Minchin ('15) 454 



2. Thomson 27 9. Minchin ('15) 446 



3. Thomson 37 10. Minchin ('15) 449 



4. Pearl 35 11. Oshorn 92 



5. Pearl 3 12. Osborn vii 



6. Castle 6 13. Osborn 93 



7. Minchin ('15) 450 14. Osborn 21 



