g6 THE DOCTRINE OF DESCENT. 



be demonstrated, not the direction of breadth, but of 

 height and depth. This mutability of the Spongiadse 

 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 

 cautious we must be in accepting proofs, since later 

 observers have sought in vain for the regular succession 

 of strata that was said to exist, and the series of modi- 

 fied forms of Planorbis multiformis contained in them; 

 indeed, 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. 



