220 The American Geologist. October, looi. 
change a single individual annelid or one restricted form of 
annelid started the process. However conceived, a number of 
forms or annelidan individuals gradually began this curious 
change, and it can be safely emphasized that such a change 
was not begun under the impulse of Jiafurol selection, since 
the turning under of two, three or four segments (somites) of 
a worm-like organism, so as to bring its mouth against its ven- 
tral surface would hardly prove a favorable position for com- 
petition with more extended and pliant congeners. 
If the inflection of the annelid body began at several dis- 
tinct localities simultaneously the stages of evolution may not 
always have been the same ; were not likely to be. And if this 
was so, there seems to be no need of presupposing even a geo- 
graphical connexion between separated geological areas, hav- 
ing similar species of trilobites ; as the evolutionary stage 
might have, must have, eventuated at many places or more 
than one place, at the same time. The hypothesis of a single 
annelidan ancestor for the trilobites, or even many identical 
ancestors seems inadvisable, and the hypothesis of a single 
center of origination and distribution even more so. 
But examining the Cambrian trilobites the impression 
deepens that the influence of environment and habits is dis- 
cernible, and that in the case of DikelocepJialits, Aglaspis, and 
Crepicephaliis, something like natural selection is hinted at. 
For in the first instance, that of the influence of environment 
and habits, it is seen in the assumption of the broad cephalic 
and genally roiinded shield. The first form of the cephalon of 
the involuted worm would naturally have been in an inflated 
rhombic outline and the gradual development of an expanded 
crescentic and more flattened shield an adaptation to its new 
habits and new surroundings. It is further safe to assume 
that the evolution of a trilobite front a worm, began in the 
larval stages of the latter in order to preclude the almost im- 
possible thought of a fixed mature organism undergoing 
changes that would so react on its own embryology as to es- 
tablish there a new series of metamorphoses. Take for in- 
stance the larval stage of the worm Polygordins, when from 
the enlarged cephalic mass the segmented body elongates, does 
not a condition stipervene which might readily give or has 
given rise to a trilobitic phase? 
