OF LIVING FORMS; OR, MORPHOLOGY. 103 



grow determine the form of living things, requires, 

 like all laws, to be seen in its relations. It does 

 not, of course, operate alone. The expanding germ 

 is moulded into its shape by the resistance it meets ; 

 but the expansion has its own laws, and does not 

 always take place equally in all directions. For 

 the most part, in growing organisms, the tendency 

 to growth exists more strongly in some parts than 

 in others; and this varying tendency depends on 

 causes which, though they are sometimes discover- 

 able, are not always so. Let us revert to the case of 

 the dorsal plates before referred to (Figs. 12 and 13). 

 If they are caused to rise up by the expansion 

 of the germinal membrane within its unyielding 

 capsule, it is evident that this membrane must be 

 growing chiefly in one direction (that at right angles 

 to their length). It is the same in almost every 

 case, but this one instance will suffice. Now this 

 tendency to growth in particular directions is some- 

 times merely apparent, and arises from these being 

 the directions in which there is least resistance to 

 expansion. Sometimes, however, it seems to be due 

 to a greater intensity, in certain parts, of the forces 



