OTHER FORMS OF DEVELOPMENT. 20I 



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 pe- 

 culiar to it. 



With respect to the other fundamental forms of de- 

 velopment, we may refer to the third chapter. The con- 

 struction of the higher Articulata points to annulose pro- 

 genitors, 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 compared 

 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 pre- 

 serve through life an unsegmented vertebral column, not 

 separable into single vertebrae, are raised as embryos 

 from this condition into their higher and definitive 

 phase. The fact that they pass through this common 

 embryonic condition excludes 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 dififerentiated 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 development, and the course, within these 

 types, from the undifferentiated to the special; but by 

 the words " type of development," the fact is para- 

 phrased, not explained; 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. 



