May 10, 1912] 
favor of the celenterate affinities of. the 
sponges with the actual structural composition 
of Spongilla and Leucosolenia, and reached 
the conclusion that the relationship of the 
sponges to certain flagellate Protozoa was so 
distinct and decisive as to forbid their logical 
inclusion among the representatives of any 
other class. The universal and characteristic 
collared cells of sponges point emphatically to 
a choanoflagellate ancestry and, as a recent 
authority has observed, in the present state of 
our knowledge it would be difficult to frame 
a definition of the Protozoa which should ab- 
solutely exclude the sponges. His work on 
flagellate Protozoa and sponges was a valuable 
contribution to science. Since his time great 
advance has been made in our knowledge of 
the histological structure, mode of reproduc- 
tion and embryological development of these 
organisms, due mainly to the opportunities 
offered by novel and refined methods of tech- 
nique. Had the art of making thin sections 
and the staining of tissues been known in his 
day, he would certainly have anticipated much 
of the later work of cytologists and embryol- 
ogists. 
In the use of the microscope, Clark showed 
not only mechanical skill and ingenuity, but 
@ patience, caution, and experience in difficult 
points in histology, which undoubtedly placed 
him at the head of observers in this country 
and rendered him perhaps inferior to few in 
Europe. He used the highest powers with a 
skill that few if any living observers have sur- 
passed. He suggested improvements carried 
out by Spencer and Tolles in this instrument. 
In 1857 Professor Agassiz sent him to Canas- 
tota to confer with Spencer, and as a result 
a microscope was made by him which was 
fully equal to any made at that time in 
Europe. Clark suggested that we must have 
three kinds of objectives: one with the field 
extremely flat; another, an immersion lens— 
the first made, so far as we are aware, and 
now so universally used; and a “third with a 
depthing focus extending as far as possible be- 
yond that of the ordinary kind, for the pur- 
pose of viewing objects as a whole, in order to 
SCIENCE 
fessor Clark, the 
729 
ascertain the relations of their different parts.” 
‘This microscope was in use in 1859. 
In 1878, five years after the death of Pro- 
Smithsonian Institution 
published, as one of its Contributions to 
Knowledge, his monograph of the “ Lucer- 
nariz and their Allies.” This group was the 
subject of his last studies, though one which 
had early engaged his attention. Complete 
and elaborate as it is so far as it extends, this 
beautiful memoir is only a fragment of what 
was evidently designed to cover at least fifteen 
parts, two parts only having an actual exist- 
ence. It has been well said that a broken shaft 
would represent both the author’s life and 
this posthumous work, each symmetrical and 
thoroughly finished to the point where they 
suddenly broke off. 
The Lucernarians were not regarded by 
Clark as truly radiate animals, but in a de- 
gree bilateral, with a fore and hinder end. 
The commonly received theory at that time 
that the so-called Radiata are founded upon 
the idea of radiation, was combated by him 
in 1865. The views of the present day coin- 
cide in the main with his—that radiation is, 
on the whole, a superficial feature, not always 
constant in Cuvier’s Radiata, though often 
well marked. He regarded the so-called 
polymorphic individuals: as “organs under 
-various disguises,” and he ascribed a high 
degree of individuality to the jelly-fish, 
Pelagia, and only a less amount to Lucernaria. 
He believed with the advanced histologists of 
-his day that “cells so-called (no matter 
‘whether constituted according to the older 
histologists or according to the most recent 
theory) are, after all, of secondary importance, 
and that the cytoblastema or protoplasm 
(which we do not distinguish from inter-cel- 
lular substance) is in the main an essential 
‘element, the potential progenitor of all tissues, 
‘and that it projects itself into the utmost 
feature of the living body by a process of self- 
proliferation. Through this, and this only, 
‘can a true law of continuous development be 
‘illustrated; while the various forms of cell- 
tissue, and fiber-tissue, and bone-tissue, etc., 
are but the disjointed collateral developments, 
