MENDELISM AND OTHER METHODS OF DESCENT 221 



Mendelism can not be denied. Indeed, the relations appear to 

 be still more intimate than even the advocates of the two theories 

 have supposed. Varieties which have arisen by mutation may 

 not only behave in a Mendelian manner when crossed, but 

 mutations may actually represent Mendelian phenomena, at the 

 time of their first appearance. 



The nature of the protoplasmic combinations and readjust- 

 ments which take place during the process of chromatin fusion 

 (mitapsis) is not yet known, but the indications favor the com- 

 monly accepted view, that new variations usually occur at 

 mitapsis. As mutations are relatively infrequent phenomena, 

 it would only be in the rarest cases that two newly mutated 

 gametes would meet in conjugation. The general rule must 

 be that new variant gametes conjugate with gametes of the 

 unmutated parental type, and thus have from the first the repro- 

 ductive status of Mendelian crosses. 



There are two ways for such a new gamete variation to gain 

 visible expression in a plant or animal. If the mutation be 

 dominant, it can make itself manifest in the first generation. 

 If it be abeyant, or recessive, expression can be secured only 

 in later generations, when it becomes possible for two of the 

 recessive gametes to be brought into conjugation. 



Mutative variations which have arisen under conditions of 

 restricted descent tend to disappear when they are brought into 

 contact with the wild type or with a less inbred stock, but new 

 mutations are often dominant and prepotent when bred within 

 their owm group of immediately related organisms. Among 

 Professor Davenport's hybrids the fiizzled plumage was domi- 

 nant, while the silk}' plumage was recessive. 



That mutations may be hybrids already, at the time of their 

 first expression in the conjugate organism, would explain dif- 

 ferences of behavior between different kinds 6f new variations. 

 A dominant variation would not be represented in all the progeny 

 of the second generation. Even though the new variation were 

 a plant which could be fertilized by its own pollen, at least a 

 quarter of the second generation would express the character 

 of the recessive gamete grandparent. 



Recessive variations, though they can not secure expression 



Proc. Wash. Acad. Sci., July, 1907. 



