94 THE DOCTRINE OF DESCENT. 



whereas the habit of the series of individuals is incredi- 

 bly variable. This variability of the whole body is not 

 lacking in the silicious sponges ; in the genus Tedania, 

 for instance, established by Gray from some of my earlier 

 Reniera, we see how their stubbornly coherent needle-like 

 forms recur from Trieste to Florida and Iceland, under 

 the most heterogeneous disguises. In some varieties, how- 

 ever, 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 absiirdiim, 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 



