i66 Experimental Zoology 



handedness and left-handcdness in man belongs to the same 

 category. It is also known that in man all of the viscera may be 

 in a reversed position, and the aortic arch also. 



Lang has bred together some left-handed snails of the right- 

 handed species, Helix pomatia. They produced only right- 

 handed individuals. These were also inbred and produced only 

 right-handed young. It is evident that this new character is 

 not inherited in this instance and also that it does not behave 

 as a recessive to the right-handed condition, for, if it did so, it 

 would have reappeared in the grandchildren. 



Physical difficulties seem to interfere with the union of the 

 sinestral individuals of Helix with dextral forms, so that as a rule 

 the sinestrals are precluded from breeding, unless by chance 

 they meet one of their own kind ; but even if this occurs all the 

 offspring will be dextral again, and no opportunity exists to per- 

 petuate the new race. , If, however, in other species the left- 

 handed condition should be heritable, such individuals might, if 

 once started, establish a variety alongside of the parent forms, 

 although the two forms might be prevented by their difference in 

 structure from pairing. That the left-handed condition must 

 sometimes be inherited is shown by the existence of species of 

 this sort. 



Conclusions regarding Mendelian Inheritance 



The examples of discontinuous inheritance given in the pre- 

 ceding chapters make it clear that Mendel's law accounts in 

 many cases for the results, and is therefore an invaluable acqui- 

 sition to our method of interpretation ; yet in some other cases 

 it is evident that the inheritance is not strictly Mendelian. 1 Used 

 with discretion the law may still unlock many problems, but if 

 attempts are made to force it to interpret cases that do not 

 belong to its proper field of action, especially in regard to dissocia- 

 tion in the germ-cells, harm rather than good may temporarily 

 result. 



1 The most striking exceptions are those recorded by Standfuss and by de Vries 

 (cenothera). Other cases involve blending, etc. 



