96 THE DOCTRINE OF DESCENT, 
be demonstrated, not the direction of breadth, but, of 
height and depth. This mutability of the Spongiadz 
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 mutability 
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 Wiirtemberg. 
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 
