216 MORPHOLOGICAL DEVELOPMENT. 



adjust themselves on the convex and the concave surfaces, 

 supposing the fibres of which they are composed to be ob- 

 lique, as their function requires; and it 

 is not difficult to see that when once 

 definite divisions have been established, 

 they will advance inwards as the layers 

 develop; and will so produce a series of 

 muscular bundles. Here then we have 

 something like the myocommata [or myotomes as now called] 

 which are traceable in the Ampliioxus, and are conspicuous 

 in all superior fishes. 



256. These are highly speculative conceptions. I have 

 ventured to present them with the view of implying that 

 the hypothesis of the mechanical genesis of vertebrate struc- 

 ture is not wholly at fault when applied to the most rudi- 

 mentary vertebrate animal. Lest it should be alleged that 

 the question is begged if we set out with a type which, like 

 the Amphioxus, already displays segmentation throughout 

 its muscular system, it seemed needful to indicate conceiv- 

 able modes in which there may have been mechanically pro- 

 duced those leading traits that distinguish the Amphioxus. 

 All I intend to suggest is that mechanical actions have been 

 at work, and that probably they have operated in the manner 

 alleged : so preparing the way for natural selection. 



But now let us return to the region of established fact, and 

 consider whether such actions and reactions as we actually 

 witness, are adequate causes of those observed differentiations 

 and integrations which distinguish the more-developed verte- 

 brate animals. Let us see whether the theory of mechanical 

 genesis affords us a deductive interpretation of the inductive 

 generalizations. 



Before proceeding, we must note a process of functional 

 adaptation which here co-operates with natural selection. 

 I refer to the usual formation of denser tissues at those 

 parts of an organism which are exposed to the greatest 



