1889.] On Variation. 1087 
would seem a proof !hat that the old and large genus was becom- 
ing extinct by the fixation of various structural types foreshad- 
owed in it in a general way. May it not be for this reason, and 
not because of any sudden catastrophe, that the formerly abund- 
ant genus disappeared? In the three prominent genera of tri- 
lobites which characterize the three divisions of the Cambrian 
rocks, there is an example of the entire disappearance of 
one genus before the appearance of the next. Between the 
two earlier ones (Olenellus and Paradoxides) is an intermediate 
genus or sub-genus (Mesonacis) possessing features of both, while 
the presence of a connecting link (Olenoides) between the second 
and third genera (Paradoxides and Dikellocephalus) is also prob- 
able. Then the extinction of number one (Olenellus) will be in 
reality the birth of number two (Paradoxides) ; and the dying 
ou t of that form be the beginning of the life of number three 
(Dikellocephalus). Finally, may not this last genus find its rep- 
resentative in number four (Asaphus), which characterizes rocks 
still higher in the geologic column ? 
These are suggestions, not assertions. But at the same time 
we believe it to be really true that large genera of the earlier 
geologic periods contain in themselves the elements which, later 
on in the life of the world, become well-defined generic charac- 
ters. Present only in a rudimentary form at one period, well- 
marked and distinctive characters appear at a later one. 
March 20, i8go. 
