66 MEMOIRS NATIONAL ACADEMY OF SCIENCES. [vol.xii. 



Morgan (Experimental Zoology, 1907, p. 166) states that Lang bred together left-handed 

 individuals of the right-handed species Helix pomatia, and only right-handed young were pro- 

 duced, even when they were inbred. (Cf. Lang, 1906a, p. 495.) 



Lang (1906, 1908) for many years studied heredity in Helix nemoralis and Helix hortensis. 

 These shells show great variation in shell color, banding, size, and form of the umbilicus. To a 

 large degree, similar variations take place in both species. The different colonies of these shells 

 show considerable individuahty in the kind of variations found in them. Thus some colonies 

 are composed solely of five-banded shells, and these are relatively common; others are composed 

 of the bandless individuals, which are rare; some colonies may stand anywhere between these 

 two extremes, and still others are composed of both the banded and the bandless shells, both 

 sharply defined and with no intermechate forms. 



Inbred snails from the pure five-banded colonies were found to produce only the five-banded 

 shells; they bred true. Pure bandless colonies were much more difficult to find but were also 

 found to breed true. 



When pure strains or races of the banded and bandless were crossed the young of the first 

 generation (FJ were all bandless. The bandless were dominant (d) and the banded were reces- 

 sive (r). When these hybrids of the first generation (Fj) were bred together in pairs, their 

 progeny, the hybrids of the second generation (Fj), were found to split up into the bandless and 

 banded kinds, in the proportion of three bandless (dominants) to one banded (recessive). This 

 splitting up in the second generation of the hybrids (F2) into the two grand-parental strains is 

 typical Mendelian inheritance. When the progeny of the second generation (F^) of hybrids 

 are bred together in pairs, it is found that the progeny of the banded shells give rise only to 

 banded shells, a pure race of banded shells, the extracted recessives. These are a pure race 

 extracted from the hybrid mixture. The bandless shells are found to be of two kinds, those 

 which breed true bandless shells, the extracted dominants, which form a pure strain of bandless 

 shells, and finally those (dominant-recessives) which produce bandless and banded shells in the 

 proportion of the three bandless to one banded. The entire population of this F3 generation is 

 composed of bandless and banded individuals in the ratio of three bandless to one banded. 



These relations may be made more graphic by reference to figure 60, in which lo shells have 

 been arranged as if they showed Mendelian inheritance in the same way in which Lang found 

 that it takes place in Helix. In this case the smooth shells (s) are considered dominant (d) 

 and the spinose shells (t) are considered as recessive (r). The results of such a cross are, in the 

 first generation (FJ all are smooth or dominant (d) and the spinose character is hidden or latent, 

 or it is said to be recessive (r). Pairing individuals of this generation, their young, hybrids of 

 the second (F^) generation, split into two kinds, both spinose and smooth, like the grandparents, 

 but the smooth are three times as numerous as the spinose. If these shells are bred in pairs, it is 

 found that the progeny of the spinose shells give rise only to spinose young (F3), a pure race 

 of spinose shells, the extracted recessives. The smooth shells are in this way found to be of 

 two kinds because they give rise to different kinds of progeny. One produces smooth shells 

 only (F3), the extracted dominants, a pure race of smooth shells extracted or split off from the 

 hybrid mixture, and the other (dominant-recessives) produce young (F3) smooth and banded 

 in the proportion of three smooth to one spinose. The entire F3 population is composed of smooth 

 and spinose shells which occur in the ratio of three smooth to one spinose shell. 



This splitting up of hybrids into the parental strains is considered the essential feature 

 of MendeHan inheritance, according to Bateson (1909, p. 50), who considers dominance assub- 

 ordinate and incidental. Lang (1908, p. 53) found that this splitting or segregation occurred in 

 the second (Fj) generation. Mendelian inheritance was found to apply to the following characters : 

 (1) Banding of the shell, (2) ground color of the shell, (3) size of the shell, (4) form of the tmi- 

 bilicus. He also showed (1906, p. 233) that in hybridization the varietal characters Mendelized 

 or split, as did also crosses between the species, in the same general way. Crosses between va- 

 rieties are generally fertile, while crosses between species are only so very rarely. 



In these hermaphroditic, but not self-fertilizing land shells, Lang (1908, p. 40) proved that 

 after copulation potent sperm may remain in the receptaculum seminis for years. Also that 



