52 



THE . 1 MEBICA N NA TURALIST [ Vol. XLII 



ditions, in isolated positions, by diversity in their "eco- 

 logical habits." He describes them as "cases of ecolog- 

 ical (or binomic) isolation, where no 'barriers' in the 

 ordinary sense are present." In the relations of 

 Hawaiian snails such cases are not infrequent. Even 

 when the conditions in the environment with which the 

 organism deals arc extremely simple, the divergent groups 

 of the original stock may adopt several different methods 

 of dealing with these conditions, and with marked success 

 in the use of each method. A fine illustration of this fact 

 is found in the different species of Triposolenia, a genus 

 of pelagic protozoa described by Professor C. A. Kofoid, 

 in the University of California Publications, Zoology, Vol. 

 3, Nos. 6, 7 and 8, December 11, 1906. After considering 

 the nature of the several different types found coexisting 

 in the same region, "in the surface waters of the sea," he 

 concludes that, "The utility of each of the complexes of 

 characters is sufficient for their preservation without the 

 necessity of calling in natural selection to account for 

 their differentiation and continuance." (Sec pp. 121, 

 123.) 



Causes of the Earlier Forms of Divergence 

 In the discussion that has appeared in Science during 

 the past two years, the chief interest has centered around 

 the question of the causes of the earliest forms of diver- 

 gence. One method of inquiry has been to observe 

 whether in a given family of organisms the most nearly 



form, or habits of feeding, or instincts for mating, or time 

 of flowering, or in prepotence of pollen, or in some other 

 quality producing incompatibilities, and so producing 



