OTHER FORMS OF DEVELOPMENT. 2OI 



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 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 me- 

 chanical 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. 



