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." 
Dimor phism 
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 
* Correns has described similar results in plants. 
