Likenesses ; or, Philosophical Anatomy 275 



by Professor Owen, that serial segmentation, however latent 

 and disguised, extended primitively and fundamentally to 

 quite the anterior end of the head. The first-named Pro- 

 fessor now advocates the view that we have an approxima- 

 tion to the early form of the vertebrate skull in that very 

 exceptional little fish the Lancelet (Amphioxus),'^ in which 

 the front end of the body is, like all the rest of it, made up 

 of a series of similar segments, although the part representing 

 the bodies of the vertebrae of higher animals is itself unseg- 

 mented. The general resemblance of the new concrete type 

 of Professor Huxley to the old type, as exhibited in the well- 

 known plate of Professor Owen's book on the archetype of 

 the vertebral skeleton, is striking enough. 



It is none the less true that there are profound differences 

 between the two conceptions. According to the recently put 

 forth view, the skull of the higher vertebrates is really made 

 up of something less than twenty segments, each of which 

 has a morphological value equivalent to a spinal vertebra 

 with its annexed parts. Again, the recent conception does 

 not repose so much upon a speculative basis, but presents us 

 with an apparently concrete type instead of an abstract ideal. 

 And yet even the concrete Amphioxus must after all be 

 ideaUsed to serve as the type of vertebrate structure, since in 

 ear-structure it is strangely defective, and, though its body is 

 segmented as a whole, the central part of the spinal column 



not segmented, but presents, like the embryos of the 

 ligher animals, a continuous chorda dorsalis. 



The conception of cranial vertebrae, then, like the concep- 



^ The constantly increasing number of instances of the independent origin 

 )f similar structures makes us think it far from impossible that vertebrate 

 jenetic affinity may lie at least as much in the direction of the annelid worms 

 in that of the ascidians, and that there are hardly yet data to determine 

 T'hich of the curious relationships exhibited by the Lancelet are due to 

 genetic affinity, which to homoplasy, and which perhaps merely to degrada- 



