334 PHYSIOLOGICAL DEVELOPMENT. 



classed as malignant. The blood-vessels In an enccphaloiJ 

 cancer, are led to enlarge and ramify, often to an immense 

 extent, by the unfolding of the morbid mass to which they 

 carry blood. Alien as is the structure as a whole to the type 

 of the organism ; and alien in great measure as is its tissue 

 to the tissue on which it is seated ; it nevertheless happens 

 ( hat the growth of the alien tissue and accompanying ab- 

 straction of matei'ials from the blood-vessels, determine a 

 corresponding growth of these blood-vessels. Unless, then, 

 we say that there is a providentially- created type of vascular 

 structure for each kind of morbid growth (and even this 

 would not much help us, since the vascular structure has 

 no constancy within the limits of each kind), we are com- 

 pelled to admit that in some way or other the currents of 

 blood are here directly instrumental in forming their own 

 channels. One more piece of evidence, before cited 



as exemplifying adaptation (§ 67), may be called to mind. 

 When any main channel for blood, leading to or from a 

 certain part of the body, has been rendered impervious, 

 others among the channels leading to or from this same part, 

 enlarge to the extent requisite for fulfilling the extra func- 

 tion that falls upon them : the enlargement being caused, as 

 we must infer, by the increase of the currents carried. 



Here then are facts warranting inductively the deduction 

 above drawn. It is true that we are left in the dark respect- 

 ing the complexities of the process. How the channels for 

 blood come to have limiting membranes, and many of them 

 muscular coats, the hypothesis does not help us to say. But 

 the evidence assigned goes far to warrant the belief that vascu- 

 lar development is initiated by direct equilibration ; though in 

 direct equilibration may have had the larger share in establish 

 ing the structures which distinguish finished vascular systems 



§ 301. Of the inner tissues which remain let us next take 

 bone. In what manner is difierentiated thie dense substance 

 serving in most cases for internal support ? 



