526 ANNIVERSARY ADDRESS TO THE GEOLOGICAL SOCIETY 
embryonic, or of a more generalized type, than the modern Opossum ; 
or a Lophiodon or a Paleotherium, than a modern Tapirus or 
Flyrax ? 
These examples might be almost indefinitely multiplied, but 
surely they are sufficient to prove that the only safe and unquestion- 
able testimony we can procure—positive evidence—fails to 
demonstrate any sort of progressive modification towards a less 
embryonic or less generalized type in a great many groups of 
animals of long-continued geological existence. In these groups 
there is abundant evidence of variation—none of what is ordinarily 
understood as progression ; and, if the known geological record is to 
be regarded as even any considerable fragment of the whole, it is 
inconceivable that any theory of a necessarily progressive develop- 
ment can stand, for the numerous orders and families cited afford no 
trace of such a process. 
But it is a most remarkable fact, that, while the groups which 
have been mentioned, and many besides, exhibit no sign of pro- 
gressive modification, there are others, coexisting with them, under 
the same conditions, in which more or less distinct indications of such 
a process seem to be traceable. Among such indications I may 
remind you of the predominance of Holostome Gasteropoda in the 
older rocks as compared with that of Siphonostome Gasteropoda in 
the later. A case less open to the objection of negative evidence, 
however, is that afforded by the Tetrabranchiate Cephalopoda, the 
forms of the shells and of the septal sutures exhibiting a certain 
increase of complexity in the newer genera. Here, however, one 
is met at once with the occurrence of Orthoceras and Baculites at 
the two ends of the series, and of the fact that one of the simplest 
genera, Mautzlus, is that which now enists. 
The Crinoidea, in the abundance of stalked forms in the ancient 
formations as compared with their present rarity, seem to present 
us with a fair case of modification from a more embryonic towards 
a less embryonic condition. But then, on careful consideration of 
the facts, the objection arises that the stalk, calyx, and arms of the 
palazozoic Crinoid are exceedingly different from the corresponding 
organs of a larval Comatula; and it might with perfect justice be 
argued that Actrinocrinus and Eucalyptocrinus, for example, depart 
to the full as widely, in one direction, from the stalked embryo of 
Comatula, as Comatula itself does in the other. 
The Echinidea, again, are frequently quoted as exhibiting a gradual 
passage from a more generalized to a more specialized type, seeing 
that the elongated, or oval, Spatangoids appear after the spheroidal 
