OTHER FORMS OF DEVELOPMENT. 201 
the Cephalopoda, notwithstanding their continued so- 
journ in salt water, other causes have produced the loss 
of the velum phase, and the course of development 
peculiar to it. 
With respect to the other fundamental forms of de- 
velopment, we may refer to the third chapter. The 
construction of the higher Articulata points to annulose 
progenitors, more or less corresponding to the annelids 
of present times ; and, again, the gradual increase of the 
segments of the larval annelids, which may be com- 
pared to the process of gemmation, leads from these 
higher Vermes to the lower ones with unsegmented 
bodies. All vertebrate animals, man included, if they 
do not preserve through life an unsegmented vertebral 
column, not separable into single vertebra, are raised 
as embryos from this condition into their higher and 
definitive phase. That they should pass through this 
common embryonic condition is prohibited by all other 
mechanical causes but that of a common derivation 
from primordial forms which possessed an unsegmented 
vertebral column, no cranium or an imperfect one, and 
either no brain or one little differentiated from the 
spinal cord. Karl Ernst v. Baer, who, while we write 
these pages, raises his voice against the doctrine of 
Descent, has established the fact, of types of develop- 
ment, and the course, within these types, from the 
undifferentiated to the special; but by the words 
“type of development,” the fact is paraphrased, not ex- 
plained; and, as we cannot repeat too often, we prefer 
the distinct idea of derivation to the supposition of 
an unknown higher Power manifesting itself after an 
incomprehensible fashion in the types of development. 
