CHAPTER 5 



EVOLUTION AS SEEN IN 



CHEMICAL STRUCTU RE 



AND IN METABOLISM 



Fundamental Unity of All Living Things 



In the preceding chapters we have discussed examples 

 of fundamental similarities of structure (homologies) in the gross anato- 

 mies of animals and in their embryonic developments. Homology ooes 

 much deeper than this, however. The most truly fundamental of all simi- 

 larities are similarities of chemical structure and function (metabolism). 



Chemists recognize the existence of slightly more than a hundred 

 chemical elements. All things, animate and inanimate, are composed of 

 one or more of these elements, combined together in varying ways and 

 proportions. Of these one hundred elements, living things are constructed 

 mainly of jour. These four, carbon, hydrogen, oxygen, nitrogen, constitute 

 about 99 percent of all living matter — plant, amoeba, or man. Here is fun- 

 damental similarity indeed! 



Of the remaining elements, sulfur and potassium are always found in liv- 

 ing organisms and hence, with the "big four" mentioned above, are always 

 essential to life. Still other elements are essential to most, if not all, living 

 things: copper, iron, cobalt, zinc, magnesium, manganese, calcium, sodium, 

 and chlorine. Traces of other elements are found with variable frequency 

 in the plant and animal kingdoms. These "trace elements" are frequently 

 important despite their presence in but small quantity. They often con- 

 tribute to such quantitatively small but nevertheless vital constituents as 



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