26 



THE LOCAL RACES OF THAIS LAMELLOSA 



In reviewing the various theories that have been advanced to 

 explain the possible origin of the local races of Thais, one is 

 struck by their rather contradictory character, together with 

 the lack of emphasis upon the basic factors of heredity and 

 spatial isolation. 



In the opinion of the writer the phenomenon observed in 

 the racial differentiation of 7'/?^i5 forms presents a classical ex- 

 ample of geographic speciation, paralleling the situation found 

 in many other polymorphic forms subject to some type of 

 isolating mechanism. The genetic interpretation of such cases 

 is gradually being brought to light, as is indicated by the sum- 

 mary of this subject given by Dr. Staiger in his paper on Thais 

 lapillus. In view of the close relationship between lamellosa 

 and lapillus, together with their similar type of variation, it 

 would seem highly probable that the genetic systems of the 

 two species should follow much the same general pattern. 



The variations exhibited by Thais lamellosa appear to be 

 governed by several sets of genetic elements, acting indepen- 

 dently of one another. The resultant of these factors is then 

 modified by environmental conditions, mostly of a nutritive 

 character, but with the possibility of some measure of selec- 

 tive action through the agency of predators. 



The height of the spire in proportion to the width of the 

 shell, the writer regards as determined by heredity. Popula- 

 tions with high spires appear under a great variety of ecolog- 

 ical conditions, while the same is true for low spired forms. 

 All sorts of intermediate conditions occur, but these do not 

 seem to bear any relation to environmental conditions. 



Sculpture is also regarded as determined by genetic factors. 

 The spiral ridges, so characteristic of most ot the lapilluS" 

 like forms, are as a rule feebly expressed, and may be com- 

 pletely absent; rarely they are present as a group of many 

 close-set ridges, and all sorts of intermediate conditions are 

 to be found . 



