644 A. A. W. HUBRECHT. 



d. The presence and the chemical constitution of a some- 

 times very massive intermuscular jelly by which the other 

 internal organs are at the same time surrounded. 



e. The mode of development of the mesoblast (at least in 

 Lineus obscurus), which is less specialised than in most 

 other Invertebrates. 



/. The absence of any distinct enterocoele. 

 The points of resemblance with the Chordata may be thus 

 tabulated : 



a. The general features of the nervous system. 



b. The presence of a homologue of the hypophysis cerebri as 

 a massive and important organ (the proboscis). 



c. The presence of tissues which may have become converted 

 into the notochord (viz. the material of which the proboscidian 

 sheath is built up). 



d. The respiratory significance of the anterior portion of the 

 alimentary tract. 



At the base of all the speculations contained in this 

 chapter lies the conviction, so strongly insisted upon by 

 Darwin, that new combinations or organs do not appear by 

 the action of natural selection unless others have preceded, 

 from which they are gradually derived by a slow change and 

 differentiation. 



That a notochord should develop out of the archenteric 

 wall because a supporting axis would be beneficial to the 

 animal may be a teleological assumption, but it is at the same 

 time an evolutional heresy. It would never be fruitful to try 

 to connect the difi'ereut variations o ffered, e. g. by the nervous 

 system, throughout the animal kingdom, if similar assump- 

 tions were admitted, for there would be then quite as much to 

 say for a repeated and independent origin of central nervous 

 systems out of indifferent epiblast just as required in each 

 special case. These would be steps that might bring us back a 

 good way towards the doctrine of independent creations. The 

 remembrance of Darwin's, Huxley's, and Gegenbaur's classical 

 foundations, and of Balfour's and Weismann's brilliant super- 

 structures, ought to warn us away from these dangerous regions. 



