70 THE FOUR SEGREGATIVE PRINCIPLES. 
structural selection may codperate 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 bya sinistral species, is a striking example of what De Vries 
calls mutation.* Undoubtedly new forms have sometimes arisen in 
this 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 assuined by those that cling to the underside of branches, or 
to the perpendicular trunks of trees, facilitate the production of 
young of the sinistral form? 
15. Some Young Snails have the Reverse Coil from that of their Parents. 
Since writing the above, my attention has been called to the follow- 
ing statement by Dr. A. G. Mayer,f 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 case. 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. 
+ Memoirs of the Museum of Comparative Zodlogy at Harvard College, Vol 
XXVI, No. 2, pp. 121, 122. 
