242 THE ORIGIN AND EVOLUTION OF LIFE 



gists actually observe in the characters springing from the 

 heredity-chromatin a predetermination of another kind, namely, 

 the origin through causes we do not understand of a tendency 

 toward the independent appearance or birth at different periods 

 of geologic time of similar new and useful characters. In fact, 

 a very large number of characters spring not from the visible 

 ancestral body forms but from invisible predispositions and 

 tendencies in the ancestral heredity-chromatin. For example, 

 all the radiating descendants of a group of hornless mammals 

 may at different periods of geologic time give rise to similar 

 horny outgrowths upon the forehead. This heredity principle 

 partly underlies what Osborn has termed the law of rectigra- 

 dation. Moreover, once a new character or group of characters 

 makes its visible appearance in the body its invisible chromatin 

 evolution may assume certain definite directions and become 

 cumulative in successive generations in accordance with the 

 principle of Mutationsrichtung, first perceived by Neumayr 

 (p. 138); in other words, the tendency of a character to evolve 

 in one direction often accumulates in successive generations 

 until it reaches an extreme. 



The application of our law of quadruple causes, namely, of 

 the incessant action, reaction, and interaction of the four 

 physicochemical complexes under the influence of natural 

 selection, to the definite and orderly origin of myriads of char- 

 acters such as are involved in the transformation of a shrew 

 type of mammal into the quadrupedal wolf type and of the 

 wolf type into the Zeuglodon eel type, has not yet even ap- 

 proached the dignity of a working hypothesis, much less of an 

 explanation. The truth is that the causes of the orderly co- 

 adaptation of separable and independent characters still remain 

 a mystery which we are only beginning to dimly penetrate. 



As another illustration of the complexity of the evolution 



