24 



The most surprising, and presumably unexpected result of 

 Dr. Staiger's investigation, was the discovery that the zonal 

 distribution of the ecological conditions in the intertidal area 

 was paralleled by a zonation of the several types of genetic 

 forms of Thais. The outer wave lashed zone was apparently 

 occupied almost exclusively by lapillus with germ cells carry- 

 ing 13 chromosomes, while those in the relatively undisturbed 

 interior waters bore mainly snails bearing cells with 18 chromo- 

 somes. In the intermediate tidal areas various gradational 

 forms were found between those of the outer zone with its 

 pure 13-chromosome type, and the inner zone with its popu- 

 lations with a strictly i8-chfomo8ome organization. 



This zonal distribution of the genetic isolates of Thais sug- 

 gests a selective factor affecting the populations, but Staiger 

 offers no suggestion as to what this factor might be. He de- 

 scribes it as a case of "balanced polymorphism." 



It will be recalled that Colton (1916) observed what he re- 

 garded as a zonal distribution of Thais'm regard to coloration, 

 which he interpreted as related to the selective activity of 

 predatory birds and fishes. Similarly Moore (1936), observing 

 a similar phenomenon upon the English coast, advanced evi- 

 dence to prove that the differential in coloration was due to 

 the type of food consumed by the snails. It would be of great 

 interest to learn whether these two cases, and others like them, 

 exhibit the same type of "balanced polymorphism" Staiger 

 found upon the coast of Brittany. At the time of writing Dr. 

 Staiger is entering upon a study of the genetic organization of 

 the forms of Thais found upon the Pacific Coast. The results 

 of this investigation should prove interesting, since they are 

 likely to throw considerable light upon the relationship of the 

 North Pacific forms to those of the North Atlantic. A major- 

 ity of those who have studied large scries of these forms are 

 of the opinion there must have been an interchange of faun- 

 istic elements between the two oceans during the late Tertiary. 

 It is well known that during this phase of geologic history 

 there were momentous changes, both in sea level and climate. 



