STRUCTURAL RELATIONSHIP 291 



to Aristotle, a red fluid that escaped from an organism 

 when injured. That blood owed its redness to corpuscles, 

 and that their color in turn depended upon the quantity 

 of hemoglobin they contained were facts beyond his 

 power of finding out. He must have confused any 

 other red fluid with blood had he found it. So soon as 

 it was discovered that the redness of the blood depended 

 upon the hemoglobin and that this substance appeared 

 regularly and in large quantities in some of his lowest 

 groups, the differential value of the character was lost. 

 What is true of Aristotle's may be true of any differential 

 factor in classification with expanding knowledge 

 its validity may be destroyed. The problem of the 

 systematist is to find the invariable characters and 

 build upon them. 



The purpose of classification may differ, and therefore 

 the means may also differ. The earliest classifications 

 were supposed to partake of the nature of a family tree 

 and were based upon the supposition that the higher 

 forms of existing animals arose from the lower forms by 

 some more or less direct mutation. 



Ancient observers had not overlooked the fact that 

 bones and teeth of extraordinary size and remarkable 

 shape were found here and there upon the earth's sur- 

 face, and that beds of marine shells were sometimes to 

 be found upon the uplands and in the mountains. 

 These puzzling circumstances were generally accounted 

 for upon the supposition that in the prehistoric times 

 giants and chimerical monsters had inhabited the earth, 

 and that there had been great deluges when the sea 

 arose and covered the highest mountain tops. Myths 

 to account for such things are to be found among all 

 nations. Later scientific scrutiny showed that these 

 " fossil" remains for the most part resemble living 

 creatures, though some are dissimilar. Gradually 

 Geology, the study of the formation of the earth's crust, 

 and Paleontology, the study of extinct forms of life, 

 developed as special departments of investigation, receiv- 



