54 Presence and Absence [ch. 



due to the presence of a definite factor, while the corre- 

 sponding recessive owes its condition to the absence of the 

 same factor. This generahsation, which so far as we yet 

 see, is appHcable throughout the whole range of Mendelian 

 phenomena, renders invaluable assistance in the interpreta- 

 tion of the phenomena of Heredity. The green pea, for 

 instance, owes its recessive greenness to the absence of the 

 factor which, if present, would turn the colouring matter 

 yellow, and so forth. With the examination of further 

 evidence the significance of this principle will become 

 readily apparent 



Mendel's System distinguished from that of Galfon, 



From the outline of the evidence now set forth the 

 essential aims and methods of Mendelian inquiry will have 

 been understood. By this method we reach reality and 

 concrete fact among phenomena that had become almost 

 proverbial for their irregularity. The key to the problems 

 of genetics and, as we confidently believe, to that of Species 

 also, lies in the recognition of the character-units, ox factors 

 as we often call them. Their allelomorphism is a pheno- 

 menon of gametogenesis, and is a consequence of those 

 attractions and repulsions by which the germinal cell- 

 divisions are effected. Discontinuity in variation — to use 

 the word variation in its old, comprehensive sense — results 

 from the existence of these units. We recognize therefore 

 that this discontinuity — Galton's "Organic Stability" — is 

 ultimately dependent on the physiology of gametogenesis, 

 and not as we formerly supposed on some feature in the 

 physiology of zygotes. How this simple conclusion was 

 missed we may in vain surmise. The discovery at one 

 stroke replaces all previous disquisitions regarding the laws 

 of inheritance. The magnitude of the discovery and the 

 novelty of its consequences have indeed delayed general 

 recognition of its truth. To this may have been due the 

 curious fact that the famous Nageli failed altogether to 

 realise the importance of Mendel's work. Nageli was of 

 course especially devoted to the study of heredity, and even 

 made it the subject of elaborate mathematical treatment. 

 As we now know, he was in correspondence with Mendel, 



