NO. 2.] SNAILS OF THE GENUS 10— ADAMS. 67 



if a snail has already copulated with one of its own species, and later copulates with an individual 

 of another species, it is only the older sperm of its own species which fertihzes the eggs. 



Certain banded varieties were found to be characteristic of hortensis and nemoralis (1906, 

 p. 253). 



Lang's (1906, 1908) observations on dominance are of much xalue. He has found that 

 bandlessness is dominant over banded, and red ground color over yellow (1906, p. 334), and 

 probably the less banded condition over the more banded condition. He also observed (1908, 

 pp. 54, 59, 77) that when a yellowish shell is crossed with a red or brown one, the young shell 

 is at first yellow and later, as growth progressed, the red or brown color became fully dominant. 

 This change of dominance with age has been observed in widely different organisms. 



If two varieties are crossed, each having different dominant characters, the hybrid off- 

 spring will contain both dominant features in distinctness. There is a mixing but not a blending. 

 Thus if (1906, p. 234) a yellow bandless variety of nemoralis is crossed with a red banded one, 

 the young will be red and bandless. The dominant red is derived from one parent and the 

 dominant bandlessness from the other. ' 'They are indeed not intermediate forms, but mixed 

 forms," says Lang. 



The same kind of characters which go to make up individual variations Lang (1906, pp. 

 248-250) found to be inherited. Here is seen the basis for such differences as are fovmd in colo- 

 nies and in many geographic variations. Some colonies contain widely distinct types while 

 in others all intergradations exist between these. 



On account of the inheritabihty of large and slight variations, Lang considers that herita- 

 biUty of a character rather than its abruptness and distinctness is the better criterion of a 

 mutation. He (1906, p. 253) thus holds that there is in reaUty no fundamental distinction be- 

 tween variations and mutations. Holmes (1909, p. 283) has expressed a similar opinion about 

 mutations : 



About the only criterion by which they may be recognized is their stability, and even that gives some evidence 

 of being a matter of degree. No limit has been discovered to the minuteness of the stable modifications that will occur, 

 and it may happen that further study will reveal the comparatively frequent appearance of very slight variations of 

 this kind. 



Lang 's studies of Helix are epoch making in their bearing upon the problems of differentia- 

 tion in snails. They show, as never before, the need of further pedigree experiments, the value 

 of detailed studies of snail colonies, local and geographic races, and the need of a careful analysis 

 of the conditions which determine and influence the conditions of breeding. 



The manner of inheritance, of shell color and banding, in Helix hortensis and memoralis 

 at once raises the question as to whether or not this same kind of inheritance does not apply 

 to the Acitinellidae of the Hawaian Islands. According to GuUck (1905, pp. 37-43) the island 

 of Oahu is 40 miles long, and alone possesses from 200 to 300 species, and over 1,000 varieties 

 of these shells. Great numbers of them occupy areas of only a few miles in extent, or are limited 

 to single valleys. They show a great amount of intergradation. One of the latest studies of 

 these shells is by Borcherding (1906), but it has been conducted along the older purely taxonomic 

 lines with no attempt to relate his results to the general conclusions of Gulick's work or to 

 Lang's earher work (1904). Hyatt (1898), and Hyatt and Pilsbry (1911) have discussed their 

 migrations and zoogeographic relations, but also without regard to Lang's studies. 



The probabilities appear to be that Mendelian inheritance holds for these shells, much as 

 in Helix. It also appears probable that the same conditions will be found to apply to the genus 

 Partula, investigated by Mayer (1902) and Crampton (1907, 1909). Crampton and Mayer agree 

 with Gulick that the environment can not be the determining factor in producing the varietal 

 and specific differences. Both Crampton and Mayer favor "mutation," evidently in the De 

 Vriesian sense, as contrasted with that of Lang. In the light of Lang's experiments, and his 

 idea of the inheritance of all degrees of variation and mutation, his use of the term mutation 

 seems much more applicable. 



Color differences form a conspicuous feature in the variation of the Achitinellidae, as in the 

 Helices studied by Lang. lo also possesses color bands which are very variable in their number 



