160 Experimental Zoology 



Lang has studied the hybrids between the closely related 

 species of Helix hortensis and Helix nemoralis. These species 

 are very similar, and it has often been disputed whether they 

 are separate "species." They differ principally in size, in the 

 form of the peristome of the shell and in the color of the lip, in 

 the form of the "dart" and in the finger-formed gland. The 

 hybrids are infertile with each other. Within the limits of each 

 of these species there are the same kind of varietal differences, 

 and these, as pointed out in the previous chapter, dominate and 

 recede in the first species-cross. It is not possible to test their 

 further behavior, since the species-hybrid is infertile. In re- 

 gard to other characters, Lang states that these also dominate 

 and recede. The hybrids are not intermediate, but have the 

 form of the peristome of H. hortensis and the pigmentation of 

 the lip and of the throat of the shell of H. nemoralis. The 

 dart and the finger-shaped gland are exactly those of H. hor- 

 tensis. Here it is evident that the hybrids are mixed, but 

 that in some characters they are true to one species and in others 

 to the other species. If the numbers of such characters were 

 larger, the hybrid might appear to be a blend of the different 

 characters, while in reality it might be only a mixture of one and 

 the other parental characters. It is evident that the charac- 

 ters must be studied separately in such cases before we can con- 

 clude whether species-hybrids show blending of the parental 

 characters or whether they give mixtures (mosaics) in their char- 

 acters. Some characters may blend, others alternate in their 

 inheritance. 1 



Dimorphism 



The word "dimorphism" is sometimes used for cases in which 

 the male and female differ markedly in form, but it is also used 

 for those cases in which two forms of the same sex exist. I shall 

 use the term here in the latter sense. Few cases of this sort exist 

 amongst animals, and no experiments have been made to test 

 the inheritance. The male hercules beetles occur under two 



1 Correns has described similar results in plants. 



