1892.] 



THE BRITISH NATURALIST. 



71 



continued with the progeny of each, in order to ascertain whether the 

 hybrids [i.e., mongrels] are as fertile as the pure-bred varieties. 

 In his elaborate works upon this subject, M. Jordan describes and fig- 

 ures an immense number of constant varieties within the limits of this, 

 that, and the other species — varieties which differ from one another in 

 such minute points of morphological character that they can only be 

 detected by careful observarion, yet, when detected, the varieties are 

 found by experiment to be mutually sterile. " Now, in this country, 

 no one, excepting myself, appears to have taken up this very 

 interesting line of research . . . ." In testing this point in 

 Helix nemonilis, it would be desirable to ascertain whether the difficulty 

 of mating between different varieties is due to a physiological barrier, 

 as above indicated ; or to a psychical tendency, resulting in the 

 association of " like to like." In either case the result would be the 

 same, but there is a radical difference between the two things. I 

 do not quite understand from Mr. Gain's notes, whether he failed to 

 get diverse varieties to unite, or whether the union between them 

 failed to produce ^'■oung. In the latter case the barrier would be 

 certainly physiological ; in the former it might be either, or both. 

 Scattered observations on the breeding of Helices are fairly numerous, 

 and I need not attempt to enumerate them ; but these points can only 

 be decided satisfactorily by prolonged and careful experiments with 

 numerous individuals. Helix hortensis has long been known to occur 

 on islands off the New England coast, where the specimens in each 

 locality are (unless with rare exceptions) alike in colour and banding. 

 Dr. Amos Binney, in the second volume of " The Terrestrial Air- 

 Bveathing Mollusks of the United States,'' p. 114, remarks "When I 

 first visited Salt Island, where this species abounds, ten years ago, it 

 was impossible to find a single specimen with either lines or bands. 

 One uniform colour prevailed throughout. At the present time, the 

 banded varieties are said not to be uncommon." To this, Gould adds, 

 " They have recently been discovered by Dr. Samuel Cabot, in great 

 numbers, on House Island, another of the little islets in the vicinity 

 of Cape Ann, where all of them are of the banded variety. On the 

 outer Gooseberry, another neighbouring islet, he found still another 

 variety." — This was published in 1851. Recently, Mr. J. H. Thomson 

 sent the present writer a box-full of H . hortensis from the Island of 

 Martha's Vineyard, U.S.A., and these were all plain bandless yellow. 

 The explanation of tliese interesting cases is, I take it, that only one 

 variety was originally introduced in each island, and this has kept 

 pure by isolation. The fact thai colonies on the mainland are not so 

 uniform in colour is well-known ; thus, I received six varieties from 

 one locality in Massacluisetts. Helix iicjiioral is, on being introduced 

 into Virginia, has shown the most extraordinary amount of variation, 



