94 THE DOCTRINE OF DESCENT. 
whereas the habit of the series of individuals is incredibly 
pliable. This pliability of the whole body is not lacking 
in the silicious sponges; in the genus Tedania, for in- 
stance, established by Gray from some of my earlier 
Reniera, we see how their stubbornly coherent needlc- 
like forms recur from Trieste to Florida and Iceland, 
under the most hetcrogeneous disguises. In some varie- 
ties, however, one of these spicula already manifests a 
tendency to deviations. 
This very point, the possibility of tracing in detail 
the metamorphoses of organs, which, on the assumption 
of their stability, appeared to provide the system with 
the most substantial basis for the erection of genera and 
species, renders the investigation peculiarly attractive. 
Even among the Algierian sponges, I have adduced 
striking examples, and they accumulate in proportion 
as the horizon is extended. We arrive gradually at the 
conviction that no reasonable dependence can be placed 
on any “characteristic ;” that with a certain constancy 
in microscopic constituents, the outward bodily form, 
with its coarser distinctive marks, varies far beyond the 
limits of the so-called species and genera; and that, 
with like external habits, the internal particles, which 
we looked upon as specific, are transformed into others, 
as it were, under our hands. “Any one”—thus con- 
cludes this section of my work on the Fauna of the 
Atlantic Sponges,—* who, with regard to sponges, 
makes his chief business the manufacture of species and 
genera, is reduced ad absurdum, as Haeckel has shown 
with exquisite irony in his Prodrome to the Monograph 
on the Calcareous Sponges.” 
In my specific researches I confined myself essentially 
