258 DE. C. CHILTON ON THE SUBTEEEANEAX 



variety described by Schneider, forming to some extent a link between this variety and the 

 surface-type, though ajjproachiug again towards Gammarus fluviatilis, while Schneider's 

 A'ariety seems to come from Gammarus pulex [78, p. 40]. These examples are sufficient 

 to make it clear that the subterranean fauna of any country is not always to be looked 

 upon as a whole, but that the different species have in many cases adopted the subter- 

 ranean life at different times, and that under favourable circumstances the subterranean 

 forms may even at the present time be reinforced by immigrants from the surface. It 

 is, however, obvious that transitional forms of the kind spoken of above will be much 

 more common in caves than in underground waters, for the means of entrance will 

 usually be greater, and immigrants from the siu-face will have to struggle with the forms 

 that have already become adapted to a subterranean life, and will hence find it difficult 

 to establish themselves unless they should happen to be carried into regions not already 

 peopled. This is probably the reason why the subterranean Amphipod fauna of Europe 

 is so fixed in character and uniform as it is, and why the subterranean species sometimes 

 belong to such ancient forms. 



In the case of the New-Zealand forms it is rather rash to speculate, but, considering 

 the similarity of conditions all over the Canterbury Plains and the fact that all the 

 known species, with the exception of Phreatoicus assimllis, are found together in the 

 same stratum of water at Eyreton, it seems jn-obable that all the forms adopted the sub- 

 terranean life at about the same time, and that they are not now being reinforced by 

 fresh immigrants from the surface. This is not for want of opjiortunities (for I have 

 shown above, on p. 249 et seq., that there are doubtless many ways by which the surface- 

 forms can gain access to the underground waters), but because the surface-fauna from 

 which the subterranean forms were derived no longer exists in its entirety. If it still 

 exists at all it will doubtless be found preserved in mountainous situations in the same 

 way as Pherusa ccerulea and Phreatoicus australis. 



The deep-water fauna of the sea and especially of freshwater lakes presents many 

 resemblances to the subterranean fauna. The deep-water fauna of the Swiss Lakes, as 

 described by Professor Eorel [40], is particularly interesting in this connection, as it 

 contains two species at least which also belong to the subterranean fauna, viz. NipJiargus 

 Forelii (= N. p'liteanus, Koch, var. Forelli, Humbert) and Asellus Forelii, Blanc. In 

 considering the origin of this deep-water fauna Professor Forel comes to the conclusion 

 that the greater part of it is derived from the littoral fauna of the lakes themselves, the 

 animals having descended to greater or less depths, and having become more or less 

 modified accordingly, and he also points out that every year new immigrants come to 

 renew the deep-water fauna, just as we saw probably happens vaih the cave-favma. With 

 regard to the two species mentioned above, however, the case is different, and after a 

 very long and full discussion of the whole problem [40, pp. 170-183] he decides that 

 these come, not from the littoral fauna, but from the subterranean fauna that is so wide- 

 spread throughout Europe. This confirms the conclusion of Humbert, Wrzesniowski, 

 and others, that Niphargus is not the direct descendant of Gammarus pulex of the 

 surface. The latter species is found in the littoral fauna of the lakes and sometimes 

 extends to deep water; but though it may be somewhat modified as regards colour, eyes, &c.. 



