SERIES OF FORMS IN SPONGES. 93 
clusion, proved in special instances, that in this group 
of low organisms which secrete the most delicate cal- 
careous shells, there could be no question of “species,” 
but only of “series of forms.” Forms which the sys- 
tematists had reduced to different genera and families, 
he beheld developing themselves from one another. 
These Foraminifera are, however, so simple in structure, 
the history of their individual evolution or Ontogenesis 
is, as yet, so little known; they contribute so little 
microscopic detail, which might formulate the law of 
transmutation of species, that the champions of persist- 
ency of species might still seek refuge in the assertion 
that Carpenter’s series of forms are mere varieties, and 
only prove that the true “species” have not yet been 
found. 
We may now turn with advantage to the class of the 
Spongiade, the importance of which in the question of 
species I was the first to point out.* With them, as I 
summed up my researches, it is not as with the Forami- 
nifera, merely an affair of the general, habit of the form, 
of the variable grouping of the chamber systems; but the 
variability exists still more specially in the microscopic 
detail than in the coarser constituents. In the Forami- 
nifera we may speak of microscopic forms, but not pro- 
perly of microscopic constituents. But in the sponges 
we discern the transformation of the finer morphological 
constituents, the rudimentary organs, and we thereby 
gain an insight into the mutability of the whole. In this 
respect the calcareous sponges are somewhat differently 
circumstanced from the rest, and from the silicious 
sponges in particular. In the former, the variability of the 
microscopic parts is limited to a smaller circle of forms, 
