THE LAW OF EVOLUTION 



and in that place into another of its isomeric 

 equivalents, gum. By secondary change some 

 of the cellulose is modified into wood ; while 

 some of it is modified into the allied substance 

 which, in large masses, we distinguish as cork. 

 And the more numerous compounds thus grad- 

 ually arising initiate further unlikenesses by 

 mingling in unlike ratios. An animal ovum, 

 the components of which are at first evenly dif- 

 fused among one another, chemically transforms 

 itself in like manner. Its protein, its fats, its 

 salts, become dissimilarly proportioned in differ- 

 ent localities ; and multiplication of isomeric 

 forms leads to further mixtures and combina- 

 tions that constitute many minor distinctions 

 of parts. Here a mass darkening by accu- 

 mulation of haematine, presently dissolves into 

 blood. There fatty and albuminous matters 

 uniting, compose nerve-tissue. At this spot 

 the nitrogenous substance takes* on the charac- 

 ter of cartilage ; and at that, calcareous salts, 

 gathering together in the cartilage, lay the foun- 

 dation of bone. All these chemical differentia- 

 tions slowly and insensibly become more marked 

 and more multiplied." " 



The differentiations of tissues and organs are 

 equally interesting. In the growth of any exo- 

 genous stem, the outer layer, or bark, first be- 

 comes distinguished from the woody interior. 



1 First Principles, p. 334. 

 223 



