96 THE DOCTRINE OF DESCENT. 



be demonstrated, not the direction of breadth, but of 

 height and depth. This mutabiHty of the Spongiadae 

 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 mutabiHty 

 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. 



A highly instructive example of the transmutation of 

 species occurring in the lapse of time, and one which 

 may at all events be banished from the limits of varieties, 

 is offered by the Tellina (Planorbis multiformis) occurring 

 in the fresh-water chalk of Steinheim, in Wtirtemberg. 

 The deposit, derived from the Tertiary period, contains 

 the residue of a small lake, and may be divided into 

 about 40 petrographically distinguishable layers. " In 

 the whole series of strata," says Hilgendorf,^* *' the 

 varieties of Planorbis multiformis are distributed in such 

 a manner that individual layers are characterized as 

 successive strata, by the exclusive occurrence or by the 

 predominance of single or several varieties which, within 

 the layer, remain constant or slightly variable, but to- 

 wards the limits of the next layer, lead by transitions to 

 the succeeding forms. The intermediate layers furnish 

 evidence that the other forms originated by gradual 

 metamorphosis from the earlier ones ; they moreover 

 render it possible to range form to form, and to trace the 

 evolution backwards; hence it becomes manifest that 

 what above seemed distinctly divided, meets below. 

 Thus arises a pedigree richly endowed with main and 

 side branches." The forms diverge so greatly, and are 

 so constant in the main zones, which tell of periods of 



