NO. 2.] SNAILS OF THE GEISTUS 10— ADAMS. 69 



intergradations are found, we are thus further impressed with the importance of the phenom- 

 ena of inverse development. This is probably due to the crossing of the smooth and spinose 

 forms of lo. The degrees of expression of inversion are numerous, and the differences are 

 nearly as great as the differences observed between the different typical forms in the genus. 



Mendelian inheritance applies to some of the characters which show this change of domi- 

 nance with age, and this suggests the possibility that such a change may be used as a criterion 

 for the recognition of some hybrids in nature, and further as indicative of their form of inherit- 

 ance. It would greatly extend the application and utility of pedigree experiments if such a 

 criterion could be established. 



The possibility of the spreading of dominance in Mendelian inheritance is one that 

 deserves mention here. Shull ('07), Hardy ('08), and Johnson ('08) have discussed phases of 

 this question. Shull and Hardy claim that in the absence of selection dominance does not 

 tend to crowd out the recessive character, but that their ratio remains constant. Hardy states 

 that under conditions of random mating "there is not the shghtest foundation for the idea 

 that a dominant character should show a tendency to spread over a whole population, or that 

 a recessive should tend to die out" (p. 49). 



On the other hand, with selection or associative mating we should expect to find a change 

 in ratios, especially in those cases where dominance is influenced, or at least in its expression, by 

 favorable or unfavorable conditions. The opportunities for random selection are quite different 

 in colonies of different composition, even if we assume that fertility exists. Thus a smooth 

 shell in a primarily spinose population, or a spinose shell in a primarily smooth population, has 

 very different chances to get a mate of its own kind, even if they make a random choice, the 

 selection may be equally random but it is from different materials. In lo the smooth and 

 spinose shells so overlap in their geographic range that theii' chances of matuig vary much and 

 this is probably a reason why the results are so variable in different localities. Inverse develop- 

 ment is most pronounced in the localities where spinosity is well developed and both rela- 

 tively smooth and spinose shells are relatively abundant. 



Tower ('10, pp. 307-323) records that when two species of beetles, Leptinotarsa undeaim- 

 lineata and L. signaticollis, are allowed to freely interbreed for several generations, there was 

 a progressive decline of undecimlineafa until all had in the seventh generation become signati- 

 coUis and the other species had apparently been obliterated. Progeny of this fusion were 

 found to breed true and did not tend to split up. Three species were similarly combined but 

 did not do so as perfectly. The details of this process of fusion are not given, but such observa- 

 tions suggest to us how the dominance of a species over another may expand and ultimately 

 change the entire population, even though Mendelian splittiag is present, as when the female 

 of undecimlineata is crossed with the male of signaticollis (Tower, '10, pp. 297-299). In this 

 manner apparently pure strains may be formed by hybridization or fusion, or the result may 

 be due to the extermination of one member of the cross. If prolonged crossing will lead to a 

 blend and pairs of such individuals, as well as extracted dominants or recessives should be 

 isolated, they might be capable of founding new colonies and new local races, as seems to have 

 been the case in Helix. 



Even if Mendelian inheritance of spinosity should be present in some of these shells the 

 spinose shells of the Nolichucky show no recognizable tendency to split, and the same is true 

 of the dextrality of the shell. I do not recall ever having seen a sinistral individual in the 

 thousands of shells examined. 



Mendelian inheritance may also apply to the smaller degrees of spinosity, but this might 

 be much more difficult to recognize in a mixed population. But if very spiny and the very 

 smooth individuals were crossed at first the Mendelian inheritance might be so distinct and the 

 change of dominance with age so marked that it could easily be distinguished. After pro- 

 longed interbreeding the characteristics of the two forms might become so finely divided as 

 to appear as a blend. Bean ('09, p. 943) has suggested that composite types or races of men 

 ' "when crossed with opposite types follow the laws of Mendel for not many generations, then 



