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ORR. " [Vol. I. 



ent organs in one animal to be primitive in the same degree. 

 But wherever we find an organ in a condition which gives a 

 clue to its original significance, and reasonably explains the 

 largest number of variations of the organ in other animals, so 

 that we may consider the variations as secondarily derived from 

 the first condition, — such a condition would then deserve the 

 attribute " primitive." 



With this view in mind, my own work has naturally led me 

 to a consideration of the different hypotheses as to the origin of 

 vertebrates, and what bearing the evidence before me had on the 

 theories. The comparison of my own results with those of 

 other investigators suggested also explanations of some feat- 

 ures generally obtaining in the embryological period with which 

 I have dealt. I do not wish, however, to be understood as 

 considering these theories or explanations in any way per- 

 fect or ultimately satisfactory. The discussion of theories 

 of such wide bearing does not properly belong in scientific 

 description; and for this reason, and partly for the sake 

 of clearness, I have reserved these speculations for a separate 

 part. 



The hypothesis of the origin of the central nervous system 

 from a pair of dorsally converged nerve-trunks is well known. 

 It has received the approbation bf Balfour and Haeckcl ; and 

 Hiibrccht also, for a time, supported it by forcible arguments. 

 The value of the hypothesis lies therein, that it points out the 

 Platyhelminthes as the group of invertebrates from which, on 

 the one hand, by the dorsal convergence of the lateral nerves 

 may be derived the vertebrate nervous system ; and, on the 

 other hand, by the ventral convergence of the lateral nerves 

 may be derived the nervous system of Huxley s sub-kingdom 

 annulosa. This theory has beers strongly combated by the 

 theory brilliantly expounded by DoJirn and his followers, — that 

 the vertebrates are descended from annelid worms, accom- 

 panied by a process of inversion through which the ventral 

 side became the dorsal side. Some of the arguments in favor 

 of this latter theory concern the body-cavity and the distinct 

 segmentations of the body; also in the annulosa the yolk- 

 mass closes in on the dorsal side. The body-cavity is a funda- 

 mental anatomical feature in many animals, and in different 

 groups its origin seems to vary, so that it is not necessary to 



