94 THE DOCTRINE OF DESCENT. 



incredibly 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, 

 however, one of these spicula already manifests a tend- 

 ency 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 Algerian sponges, I have adduced 

 striking examples, and they accumulate in proportion as 

 the horizon is extended. We arrive gradually at the con- 

 viction 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 concludes this sec- 

 tion 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 

 to the silicious sponges, and by thousands of microscopic 



