16 NOTES AND COMMENTS [suLY 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 
inthe 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. 8. 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. Journ. Micr, Sci. xl. 1899, pp. 155-201, 8 pls.), in which he 
deals with forms called Tanganyika rufofilosa, Spekia zonata, Nasopsis 
nassa, and Bythoceras 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.” 
