DERIVATION OF THE FLORA AND FAUNA 369 



specific segregation was mainly effected after arriving at the final station, in this 

 case Hawaii. This happened yesterday or the day before, geologically spoken: 

 "the rate of erosion is such that these main islands could not have stood here as 

 they are longer than from a period late in the Tertiary" (p. 121). "Explosive 

 speciation" set in during late Pliocene and must have increased during "the great 

 Pleistocene erosion which has left such a spectacular and rugged topography in 

 its wake. New land open to colonization is conducive to speciation" (p. 122). 



This late and rapid differentiation is illustrated by ZIMMERMAN for the land 

 snails pp. 98-101 : 



Helicidae. 59 species developed from one, or possibly two original immigrant 

 stocks. 



Pupillidae. Possibly 4 ancestral species gave rise to the 86 Hawaiian forms. 



Cochliocopidae-Cochliocopinae. One immigrant of Cochliocopa stock could have 

 given rise to the 142 forms. 



C.-Amastrinae. 294 forms apparently developed from one basic stock. 



Tornatellinidae-Tornatellininae. 117 forms derived from 4 or fewer ancestral 

 forms. 



T.-Achaiinellinae. It appears certain that this subfamily had its origin and 

 development in the Hawaiian area and all of the 215 forms may have been derived 

 from a common tornatellinid ancestor. 



FOSBERG, in a chapter contributed to Zimmerman's book, tried to fix the 

 number of ancestors of the Hawaiian angiosperms. His method is quite simple: 

 if the species, few or many, of a certain genus present the appearance of a more 

 or less homogeneous group, only a single ancestor is made responsible for the 

 segregation; if subgenera or sections are distinguished, we must count with the 

 same number of ancestors as of taxonomic groups within the genus. The method 

 seems a little too easy; possibly we are confronted with a rather complicated 

 question, the solution of which I am not going to attempt. 



Setchell (^218) uttered some sensible words on migration and endemism: 



Where endemism of the degree of ordinal or family endemism occurs on oceanic 

 islands, we may feel strongly inclined to believe that evolution of such degree took 

 place on the continental area which was the source of the original migration and not 

 on the island where now found, the original becoming later extinct, leaving the migrant 

 as an endemic. The same is true of generic endemism or even specific endemism of a 

 strong type, that is when representing an isolated or aberrant species under the genus 

 (p. 874). . . . To assume that insular conditions originate new forms is to overlook what 

 has taken place on continents (p. 875). 



This is, however, what so many authors do. They claim that oceanic islands 

 follow their special laws, that a plant or an animal which happens to land far away, 

 will, as it were, lose its balance; hidden factors, repressed as long as they lived 

 on their fatherland under "severe competition", are set free and allow them to 

 develop their inherent possibilities, they are not subjected to any struggle for 

 existence in their new environment. Says ZIMMERMAN: "The environment, of course, 

 plays an all-important part in the development of species ... it is generally agreed 

 that profound changes have been effected on organisms by environment" (p. 187). 



24 - 557857 The Nat. Hist, of Juan Fernandez and Easter Isl. Vol. I 



