gf: q 
52 R. Hitchcockh—Causes of Variation. 
other. When the spiral growth of Orbiculina produces a com- 
plete circular disk, further spiral growth becomes impossible, 
and if we concede that the extrusion of the sarcode to form 
successive chamberlets is due to nutrition and growth, the 
cyclical plan then becomes a necessity. In this way it may be 
supposed cyclical growth originated, purely a result of nutri- 
tion—not by adaptation to environment, but as a result of it; 
not because such growth is or ever was better adapted to the 
conditions of life. 
‘‘We find here a steady course of variation a result of physio- 
logical processes, independent of those external causes to 
whi V accustomed to attribute such changes. These 
from the geographical and bathymetrical distribution of the 
species, ithout entering into a lengthy discussion of this 
plex species are found in the warmer waters, under conditions 
most favorable to the activity of nutritive processes. As an. 
example, the very large specimens of O. complanata from Fiji 
may be taken. On the other hand, the ancestral form 0. 
tenuissema still inhabits the colder and deeper waters, retaining 
the simple characters of its earliest known condition. 
National Museum. 
