24 Mutual Independence of Hereditary Characters 



out one phenomenon which greatly complicates the study 

 of hereditary characters. I refer to the circumstance, al- 

 ready repeatedly alluded to, of their being commonly 

 united in smaller and larger groups which behave like 

 units, the single members of the groups usually appear- 

 ing together. We see this in the staminate and pistillate 

 flowers and inflorescences of monoecious plants, in the 

 described cases of bud- variation and dichogeny. The 

 sexual characters of various individuals and the differ- 

 ence between the alternating generations of the same spe- 

 cies teach us the same thing. 



This combination of the individual characters into 

 groups is therefore quite general, although it occurs in 

 all degrees, and although some hereditary characters, as 

 for instance the power of assuming a red color, do not 

 unite, as a rule, into a group with certain others. It is 

 recognized most clearly in those cases of the formation of 

 groups of green bracts instead of flowers, caused by 

 aphids, phytopters, and other parasites, where the stimu- 

 lus calls forth a whole series of characters that ordinarily 

 develop in other parts of the plant. 



Every theory of heredity has to take into account this 

 combination of the hereditary characters into larger and 

 smaller groups, and different authors, like Darwin and 

 Nageli have strongly emphasized this point. But right 

 here lies a great difficulty which interferes with a working 

 out of the theory in detail, for in many cases it will ob- 

 viously be extremely difficult to decide whether one is 

 dealing with a single hereditary character or with a small 

 group of them. There is here a large field for morpho- 

 logical analysis which awaits working up. 



5. The Combination of Hereditary Characters 



Hereditary characters can be combined to any extent 



