g6 THE DOCTRINE OF DESCENT. 



height and depth. This mutability of the Spongiadje 

 affords the extremely important evidence that, so to 

 speak, an entire class has, even now, not attained a 

 state of comparative repose. But to confirm the mu- 

 tability of species, evidence of mutability in lapse of 

 time is justly demanded; the transition of the forms 

 succeeding one another historically in the strata of the 

 earth. 



In former editions I expressed a belief that a highly 

 instructive instance of the modification of a species in 

 time might be seen in the Planorbis from the fresh- 

 water deposits of Steinheim, described by Hilgendorf. 

 But this very example has served to warn us how cau- 

 tious we must be in accepting proofs, since later ob- 

 servers have sought in vain for the regular succession of 

 strata that was said to exist, and the series of modified 

 forms of Planorbis rnultiformis contained in them; in- 

 deed, they convinced themselves on the contrary, that 

 the unusually divergent forms of this snail occur mingled 

 throughout. However, other evidences of such modifica- 

 tion abound, and the zeal of some recent Palaeontologists 

 — Waagen, Zittel, Kayser, Neumayr, and Wiirtenberger 

 — has proved, by following up the species, so called, of 

 Brachiopoda and Ammonitidse through vast geological 

 periods, that it is impossible to arrange these large groups 

 under true species. I will allow these naturalists to speak 

 for themselves. 



Kayser concludes from an investigation of the 

 Brachiopoda of the Devonian beds of the Eifel: — "No 

 order of animals, perhaps, yields such strong evidence 

 in favour of the Darwinian theory as the Brachiopoda. 

 Any one who, like myself, has had occasion to trace a 



