212 GEOLOGICAL DISTRIBUTION. 



and in that of two or three genera of mollusks recently described by 

 Smith and Bourguignat from Lake Tanganyika Syrnolopsis, Lim- 

 notrochus, and Rumella which have an undeniably marine facies." 

 But this adaptability of animals to what might be called oppo- 

 site conditions of existence is not confined exclusively to the class 

 of Mollusca; indeed, were this the case we should have but a small 

 fragment of our fresh- water fauna accounted for. The metamorpho- 

 sis of Branchipus into Artemia, and of Artemia into Branchipus, 

 is a well-known application of the law to the class of Crustacea. 

 The existence of seals in Lake Baikal, whose salinity is practically 

 nil, is a remarkable instance of much the same kind among the high- 

 est animals, the Mammalia, while as a noteworthy offset to this, 

 coming from very nearly the lowest of all organisms, is the occur- 

 rence of marine, or non-statoblastic, types of sponges (Lubomirskia) 

 in the same lake, as well as in the Upper Congo (Potamolepis). 66 



Variation in Persistent Types. It is frequently asserted, even 

 by those who are considered competent of forming an opinion, 

 that the fact of our having in the existing faunas a number of very 

 ancient types is a proof positive against the validity of any slow 

 modification theory of descent. For, it is. contended, we are here 

 brought face to face with certain structural types which have re- 

 sisted all sensible modification during a period of millions of years, 

 and have, consequently, baffled all evolutionary tendencies towards 

 reorganisation. Thus, we have, as has already been stated, the 

 Lingula of to-day practically identical with the oldest Lingula 

 known ; the modern Nautilus, but little different from the Nautilus 

 of nearly the most ancient Paleozoic deposit, and the modern Pupa, 

 practically identical with the Pupa of the Carboniferous period. 

 A little reflection will show how illogical is tne position which 

 is here assumed. That certain species should have come down 

 to us with but slight modification from the earliest periods known 

 is about as much proof of non-modification in the group to which 

 they belong as would be furnished in the case of a house, where 

 the retention of the primitive type, the hut, might be taken in 

 evidence of non-modification in the whole class. If a want of 

 adaptation to the surroundings be the primary cause of variation, 

 then, manifestly, the rate of variation among the members of a 

 given group of animals cannot be a uniform one, for while some 

 of the members will be forced to extreme measures by reason of 



