Il6 ZOOLOGY 



either to the addition of something to a deter- 

 miner, or the loss of something, or conceivably 

 to a shifting or shuffling of what is already 

 there. Such a variation might occur in a de- 

 terminer, through some chemical change in the 

 protoplasm, and if recessive to the normal, pro- 

 duce no visible effect for hundreds of genera- 

 tions. It is therefore very difficult to say that 

 a variation is "new," in a genetic sense. Even 

 if we are sure that we have witnessed its first 

 appearance on the stage, we may not know how 

 long it has been waiting behind the scenes. 

 The discovery of multiple allelomorphs is signifi- 

 cant in this connection. These are various 

 determiners which appear to occupy exactly the 

 same place in the same chromosome, and there- 

 fore cannot coexist in a gamete. The inference 

 is very strong that these are actually mutations 

 of a single original substance. A good example 

 is found in the fly Drosophila, in which several 

 different eye colors appear to be due to modi- 

 fications of a single determiner. No gamete can 

 carry more than one of these modified factors, 

 and only two can coexist in a zygote. 

 b. Variations due to the loss of a determiner. Since 

 Bridges has shown that a fragment may dis- 

 appear from a chromosome, this type of varia- 

 tion is evidently possible. In numerous cases 

 the allelomorphs (alternative characters) are to 

 each other as plus and minus, positive and 

 negative, and this fact has given rise to the 

 "presence and absence theory." According to 

 this view, the recessive is simply the absence 

 of that which is represented by the dominant. 



