218 ON THE BRITISH SPECIES OP ENTOMOSTRACA 



we have Herrick's work on the '^ Entomostraca of Minnesota," 

 embracing not only Daphnim but all other groups of fresh- 

 water species, and various shorter memoirs by other American 

 observers. 



The Daphnice are not only a very attractive and interesting, 

 but from the point of view of the systematic zoologist, a very 

 perplexing group. Whatever series of animals or plants one 

 may take up for detailed study, he very soon finds numerous 

 intermediate and apparently incomprehensible forms which, if 

 the generally received ideas of evolution have a sufficient founda- 

 tion in fact, must be the inevitable outcome of natural processes. 

 Nowhere, perhaps, is this more plainly visible than among the 

 Daphnia^ and from this fact it arises that the number of species, 

 so-called, which have been " made in Germany" and other places 

 is so perplexingly great. Many of these species would, doubt- 

 less, tried by all morphological and physiological tests, prove 

 untenable, but there will always be differences of opinion as to 

 the values properly assignable to small structural peculiarities. 

 The form of the head in various lacustrine Daphnia affords a 

 crucial example of this. The vertex in these animals varies very 

 much in its development, some being broadly rounded and others 

 drawn out more or less extensively into a conical peak, which 

 may take on a fool's-cap or helmet shape. Upon the extent and 

 character of this development many species have been founded, 

 but inasmuch as connecting links between them may usually be 

 found plentifully, it follows that specific distinctions based only, 

 or even chiefly, on this character can scarcely be admitted as 

 valid. 



It is more than probable that a closer study of the Daphnia 

 of the British Lakes would in many cases reveal local differences 

 sufficient distinctly to characterize the inhabitants of particular 

 localities, — differences such as have been described by Sir Walter 

 Buller in the case of the lizard {Sphenodon) inhabiting various 

 islands adjoining the New Zealand coasts. Sir Walter says re- 

 specting this lizard, "It has become extinct on the mainland; 

 and it is a very curious fact that, through long isolation it has 

 become differentiated in colour in the several islands or groups 



