50 R. Hitchcock—Causes of Variation. 
ae ae that the changes observed in the shells of this: 
mily are not’ due to any inherent tendency resulting in a 
definite plan, but that they are due to causes easily understood. 
It is far from my intention to deny a definite plan of growth 
to these organisms. But plan of growth does not imply that 
there have been causes acting within the organism—special 
_ tendencies of the protoplasm toward higher structure. It 
seems to be such an assumption that has led Dr. Carpenter to 
speak of a “not understood ” progressive tendency, etc. In m 
opinion the causes of such progression as can be observed are 
easily understood; and the plan of growth becomes a natural 
consequence of these causes, which are purely Ss at 
and independent of any supposed tendency to variation 
While Dr. Carpenter, on the o and, seems to regard varia- 
the writer, on the other hand, attributes it entirely to the more 
or less favorable conditions of life of the different species. More- 
over, I am quite unable to understand how any inherent ten- 
dency to variation impressed upon the sarcode could fail to find 
expression in some differentiation of the sarcode, which in the 
cases in question hie not been observed. 
The same view seems to be held by O. Schmidt who, in his 
_Grundziige einer Spongien-Fauna des Atlantischen Gebietes me 
Dr. pong ’ 
alludes to Carpenter’s previous studies, and compares the 
changes observed in the sponges and foraminifera. He says 
Piatiles 3 in the latter are found in the general habit of the 
form and the variable grouping of the chamber- ‘systems, while 
among the sponges the variation is in the microscopic detail. 
“One may speak of the 5a form of foraminifera but — 
not of microscopic elemen 
e complexity of the Hel is merely in the multiplicity “s 
chambers and the manner of their intereommunication. 
re of growth, even in the complex O. complanata, is in all 
spects identical with that in other species, and in no essen- 
tial feature differs from that of Peneroplis. What Dr. Carpenter 
esignates as a ‘“‘ higher ac type of structure ” does not 
represent an advanced degree of specialization in any part; 
unless it results from some Path of edge which confers 
some benefit upon the organism, it seems not proper to regard 
complexity of shell-structure ee a proof of ictenien! advance- 
eking for an explanation of the cause of the increased 
Se nereas 
omplexity of shell-structure, so beautifully sitvieentied in the 
ilieline amily, the writer was led to ‘the conclusion that it is 
*. 
Se atae 
