ch. iv.] THE LAW OF EVOLUTION. 339 



its isomeric equivalents, sugar; 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 numer- 

 ous compounds thus gradually arising, initiate further un- 

 likenesses by mingling in unlike ratios. An animal ovum, 

 the components of which are at first evenly diffused among 

 one another, chemically transforms itself in like manner. Its 

 protein, its fats, its salts, become dissimilarly proportioned 

 in different localities ; and multiplication of isomeric forms 

 leads to further mixtures and combinations that constitute 

 many minor distinctions of parts. Here a mass darkening 

 by accumulation of hsematine, presently dissolves into 

 blood. There fatty and albuminous matters uniting, compose 

 nerve-tissue. At this spot the nitrogenous substance takes 

 on the character of cartilage ; and at that, calcareous salts, 

 gathering together in the cartilage, lay the foundation of 

 bone. All these chemical differentiations slowly and in- 

 sensibly become more marked and more multiplied." 1 



The differentiations of tissues and organs are equally 

 interesting. In the growth of any exogenous stem, the 

 outer layer, or bark, first becomes distinguished from the 

 woody interior. Then while the bark gradually becomes 

 differentiated into the liber, made up of woody tissue, the 

 green and corky envelopes, made up of parenchyma, and the 

 epidermis ; the interior becomes differentiated into the pith, 

 the medullary sheath, the woody layer, made up of bundles 

 of greatly elongated cells, and the medullary rays, or what is 

 called the silver grain in maple and oak. Meanwhile 

 between this heterogeneous bark and the heterogeneous 

 wood which it surrounds there appears a zone of delicate 

 cells, charged with dextrine and other assimilable matter, 

 and known as the cambium layer. At the same time 



1 First Principles, p. 334. 



z 2 



