No. 580] ORIGIN OF SINGLE CHARACTERS 231 



correlations, by means of the hormones, which are observ- 

 able in the field of physiology and medicine. Mechanical 

 correlation, as where an incipient rectigradation in an 

 upper tooth corresponds precisely with an incipient recti- 

 gradation in a lower tooth so that the upper and lower 

 teeth evolve throughout in perfect mechanical harmony 

 by the addition of character after character, affords one 

 of the most vivid instances of our total ignorance of the 

 • •miM's underlying the origin of new characters. 



9. The Separability of "Characters" in the Germ 

 The pioneer discoverer of the separability of "charac- 

 ters" in the germ was Gregor Mendel, who in 1865 exam- 

 ined seven pairs of alternating characters in hybrids of 

 two varieties of the common pea (Pisum sativum). The 

 outstanding feature of this great discovery is that the 

 separability of the "determiners" of characters in the 

 germ is far more sharply denned than the separability 

 of the corresponding somatic "characters" in the visible 

 body; as regards the "determiners" of many characters 

 it is sharp and absolute. 



A second feature of Mendel's discovery is of special 

 significance in our present review of proportional and 

 numerical characters in the hard parts of vertebrates 

 because the alternative characters which Mendel experi- 

 mented with were of both the proportional and the nu- 

 merical kind. For example, Mendel's proportional char- 

 acters of "tallness" and " dwarf ness" in the pea stalk 

 may not improperly be regarded as analogous with the 

 alternative characters of dolichocephaly and braehy- 

 cephalyof the skull of the mammal. Again, Mendel's form 

 of the pea-seed, whether round or wrinkled, may not im- 

 properly be compared respectively with the folded or 

 smooth condition of the enamel in the molar teeth of the 

 ass and of the horse, which we observe is alternative in 

 heredity. On the other hand, Mendel's definite color 

 characters, such as his flower color, whether purple, red 

 or white, or his seed color, whether yellow or green, have 

 been definitely compared by experiment and found to 



