CRUSTACEA OF NEW ZEALAND. 271 



waters have in all probability Ijeen gradually peopled by animals from the surrounding 

 neighboiu-hood, and as they advanced further and further into the darkness a selection 

 of this kind would go on in each geueration, and, as Poulton has observed, " such a 

 sifting process would certainly greatly quicken the rate of degeneration due to panmixia 

 alone" [119, p. 292, footnote]. The same explanation is quoted with approval by 

 W. P. Ball, who considers, however, that ixinmixla "would probably be the most 

 important factor in causing blindness" [3, pp. 17, 72]. 



To the various causes mentioned above we must add the effects of disuse in the 

 individual, which are undovibtedly very considerable in amount, and in cave animals 

 breeding in the darkness would commence in all cases from birth. 



I may add here one or two notes on the age of the blind fauna of caves and wells, and 

 on the rate at which development has consequently taken place in these animals. 

 Although, as I have pointed out elsewhere (pp. 2.o3-25S), there is reason to believe that 

 some, at any rate, of the blind species are older than the surface fauna at present 

 inhabiting the same neighbourhood, there seems little reason to doubt that the whole 

 underground fauna is of comparatively recent origin. Packard [82, p. 25], after con- 

 sidering the facts on the question adduced by Cope, came to the conclusion that " the 

 subterranean fauna of this country does not date back of the Quaternary Period." In 

 his later paper he repeats this opinion, and, after considering the different classes of 

 caves more fully, adds : — 



" It seems, then, fair to assume that the final completion of the caverns, when they 

 became ready for occupancy by their present fauna, may not date back more than, to 

 put it into concrete figures, from 7000 to 10,000 years, the time generally held by 

 geologists to be sufficient for the cutting of the present river gorge of the Niagara and 

 the Palls of St. Anthony. We may, then, put the age of our cave fauna as not much 

 over from .5000 to 10,000 years before the dawn of history, which itself extends back 

 some 5000 to 6000 years" [83, p. 23J. 



He concludes, therefore, that the greatest part of the cave fauna of North America 

 was directly derived from the present fauna, and that consequently the changes under- 

 gone have been brought about in at most a few thousands of years. 



The fauna of the European caves descrilied by Schiodte, &c., also seems to date from 

 the " close of the Tertiary, or more probably the beginning of the Quaternary Period " 

 (Packard [82, p. 25]). 



In New Zealand, too, the subterranean fauna must be very recent, geologically 

 speaking. All the places where subterranean forms are found are marked on Professor 

 Haast's geological map of Canterbury and Westland [53] as either " post-pliocene 

 alluvium " or " recent alluvium," most of them being in the latter. Phreatoicus, by its 

 generalized character and by its occurrence in Australia as well as in New Zealand, is 

 shown to be an ancient form, probably once widely spread in fresh waters, but of 

 course it does not foUow that its subterranean species are more ancient than the other 

 subterranean forms. If t'lorough search were made it is quite possible that some 

 species of the genus would still be found inhabiting freshwater streams among the 

 Southern Alps. 



