70 THE FOTR SEGREGATIVE PRINCIPLES. 



structural selection may cooperate in maintaining the same; but the 

 new form of selection is entirely dependent on the isolation which has 

 opened the way and partially established the new form. 



The production of a sinistral form by a dextral species, or of a dex- 

 tral form by a sinistral species, is a striking example of what De Vries 

 calls mutation.* Undoubtedly new forms have sometimes arisen in 

 1 1 lis sudden way and have been continuously propagated without the 

 aid of man in securing artificial segregation. But that it is the exclu- 

 sive or even the predominant method by which divergence of species 

 takes place has not been shown. In the case of sinistral snails the 

 question naturally arises whether there are certain habits of life that 

 favor this form. Can any reason be found why this form is much 

 more common among those groups of species that are entirely arboreal 

 in their habits than among those that live on the ground? Does the 

 position assumed by those that cling to the underside of branches, or 

 to the perpendicular trunks of trees, facilitate the production of 

 vouns: of the sinistral form? 



'O 



15. Some Young Snails have the Reverse Coil from that of their Parent*,. 



Since writing the above, my attention has been called to the follow- 

 ing statement by Dr. A. G. Mayer, t concerning certain species of 

 Partula found on the island of Tahiti : 



The young of dextral or sinistral snails are usually dextral or sinistral respec- 

 tively, but this is not invariably the ease. It is interesting to observe, however, 

 that all of the young developed within any given adult [at the same time] are 

 either dextral or sinistral, never some of them dextral and others sinistral. The 

 young are born one at a time, three eggs and two or three young snails in various 

 stages of development being often found in a single adult animal. 



This seems to indicate that the causes producing in the young a 

 reversed coil from that in the parent operate alike on each embryo 

 within the parent at the same time, and are likely to produce more than 

 one individual of the reversed form when any are produced, and so 

 to open the way for a new racial type, which we have reason to believe 

 is completely segregated from the original type, though both occupy 

 the same valley, or even the same tree. 



* An interesting article on "The Origin of Species," by De Vries, translated 

 from "Album der Nature," and revised by the author, will be found in the Pop- 

 ular Science Monthly for April, 1903. See also an article by the same author in 

 Harper's Magazine for January, 1905. 



t Memoirs of the Museum of Comparative Zoology at Harvard College, Vol. 

 XXVI, No. 2, pp. I2i, 122. 



