1 6 NOTES AND COMMENTS [july 1899 



progress in a definite direction, unless equilibrium be maintained by 

 any other equipollent factors, exhibited in the form of a differential 

 death-rate on the most fertile." 



He seeks to force biologists to face a dilemma. If the above prin- 

 ciples are accepted, then the biologist " must look upon all races as 

 tending to progress in definite directions — not necessarily one, but 

 possibly several different directions, according to the characters with 

 which fertility may be correlated — the moment natural selection is 

 suspended ; the organism carries in itself, in virtue of the laws of 

 inheritance and the correlation of its characters, a tendency to pro- 

 gressive change." If, on the other hand, the biologist does not accept 

 the principles, then he must be prepared to meet the weight of evidence 

 in the memoir. But is it not fair to remark that this evidence relates 

 to two highly artificial cases — man and the race-horse ? 



Living Fossils. 



Whether Mr. J. E. S. Moore is correct or not in his interesting hypo- 

 thesis that Lake Tanganyika represents an old Jurassic sea, and that 

 many of the molluscs in it are long-lived relicts of Jurassic fauna, he 

 must get credit for his careful and enthusiastic endeavours to make 

 good his case. We believe that there are some who are in no way 

 convinced, and it was with interest therefore that we read Mr. Moore's 

 continuation of his previous studies on the molluscs of this great lake 

 {Quart. Joum. Micr. Sci. xlii. 1899, pp. 155-201, 8 pis.), in which he 

 deals with forms called Tanganyika rufofilosa, S}jckia zonata, Nasopsis 

 nassa, and Bt/thoceras howesii, which he found on the picturesque 

 shores, or dredged from the deep waters. 



His conclusion, on which it would be unfair to throw doubt without 

 detailed criticism, is that all the evidence which has been collected 

 concerning the nature of the halolimnic Gastropods invariably points 

 to the vast antiquity of these forms. "First we have the wide dis- 

 similarity of their empty shells from those of any living types ; next 

 their rigid isolation to a solitary great lake, which, judged from what- 

 ever standard we may choose to adopt, is unquestionably of an enor- 

 mous age. Next we have the wonderful similarity of the halolimnic 

 shells now living in Tanganyika, to those which have been left fossilised 

 at the bottom of the old Jurassic seas ; and lastly, there are the 

 morphological characters of the halolimnic animals themselves, whereby 

 they become mentally depicted like nothing so much as the incom- 

 pletely developed embryos of numerous living oceanic types." 



