738 



THE THEORY OF TRANSFORMATIONS 



[CH. 



the arterial system including the heart, the central nervous 

 system of the vertebrate, including the brain itself, all begin as 

 simple tubular structures. And with them Nature does just 

 what the glassblower does, and, we might even say, no more 

 than he. For she can expand the tube here and narrow it there ; 

 thicken its walls or thin them; blow off a lateral offshoot or 

 caecal diverticulum; bend the tube, or twist and coil it; and 

 infold or crimp its walls as, so to speak, she pleases. Such a form 

 as that of the human stomach is easily explained when it is 

 regarded from this point of view ; it is simply an ill-blown bubble, 

 a bubble that has been rendered lopsided by a trammel or restraint 

 along one side, such as to prevent its symmetrical expansion — 

 such a trammel as is produced if the glassblower lets one side of 

 his bubble get cold, and such as is actually present in the stomach 

 itself in the form of a muscular band. 



We may now proceed to consider and illustrate a few permu- 

 tations or transformations of organic form, out of the vast 

 multitude which are equally open to this method of inquiry. 



We have already compared in a preliminary fashion the 

 metacarpal or cannon-bone of the ox, the sheep, and the giraffe 

 (Fig. 354) ; and we have seen that the essential difference in form 



between these three bones is a matter 

 of relative length and breadth, such 

 that, if we reduce the figures to an 

 identical standard of length (or identical 

 values of y), the breadth (or value of 

 x) will be approximately two-thirds 

 that of the ox in the case of the sheep 

 and one-third that of the ox in the 

 case of the giraffe. We may easily, 

 for the sake of closer comparison, 

 determine these ratios more accurately, 

 for instance, if it be our purpose to 

 compare the different racial varieties 

 within the limits of a single species. 

 And in such cases, by the way, as when we compare with one 

 another various breeds or races of cattle or of horses, the ratios 



