4 THE DATA OF BIOLOGY. 



chief organic elements, and that comparative readiness dis- 

 played by organic matters to undergo those changes in the 

 arrangement of parts which we call development, and those 

 transformations of motion which we call function. 



Considering them chemically instead of physically, it is 

 to be remarked that three out of these four main components 

 of organic matter, have affinities which are narrow in their 

 range and low in their intensity. Hydrogen combines with 

 comparatively few other elements ; and such chemical energy 

 as it does show", is scarcely at all shown within the limits of 

 the organic temperatures. Of carbon it may similarly be said 

 that it is totally inert at ordinary heats ; that the number of 

 substances with w^iich it unites is not great; and that in 

 most cases its tendency to unite with them is but feeble. 

 Lastly, thi^ chemical indifference is shown in the highest 

 degree by nitrogen — an element which, as we shall hereafter 

 see, plays the leading part in organic changes. 



Among the organic elements, including under the title 

 not only the four chief ones, but also the less conspicuous re- 

 mainder, that capability of assuming different states, called 

 allotropism, is frequent. Carbon presents itself in the three 

 unlike conditions of diamond, graphite, and charcoal. Under 

 certain circumstances, oxygen takes on the form in which it 

 is called ozone. Sulphur and phosphorus (both, in small 

 proportions, essential constituents of organic matter) have 

 allotropic modifications. Silicon, too, is allotropic ; while 

 its oxide, silica, which is an indispensable constituent of 

 many lower organisms, exhibits the analogue of allotropism 

 — isomerism. And even of the iron which plays an active 

 part in higher organisms, and a passive part in some lower 

 ones, it may be said that though not known to be itself allo- 

 tropic, yet isomerism characterizes those compounds of it that 

 are found in living bodies. Allotropism being interpretable 

 as some change of molecular arrangement, this frequency 

 of its occurrence among the components of organic matter, 

 is significant as implying a further kind of molecular mobility. 



