ORIGIN OF THE LIVING MACHINE: ADAPTATION 359 



Mendel's Law. Accompanying the development of the the- 

 ory of mutation, there has been brought prominently to view a 

 somewhat new view of the laws of heredity, perfectly consistent 

 with Weismann's theory, but explaining its method of action. 

 Darwin in his discussion assumed that the offspring of two 

 parents, since it could not be like both, would, in general, be 

 halfway between the two. Even the slightest familiarity with 

 the laws of heredity is enough to show that organisms inherit 

 from both parents, and it has generally been assumed that they 

 inherit, or may inherit, equally from both. It is, however, 

 manifestly untrue that the offspring is always midway between 

 its father and mother, inheriting equally characters from each. 

 The laws of heredity are much more complex than this, for it 

 frequently appears that an organism inherits mostly from one 

 parent, the characteristics of the other hardly- reappearing in 

 the offspring. An attempt to bring some of these facts into a 

 general law has resulted in what is called Mendel's law of 

 heredity. Mendel published the result of his work originally 

 in 1866, but it attracted no special attention for nearly forty 

 years, when it was revived by modern students in 1900. Since 

 that time it has been subjected to extensive experiment, and 

 has produced results of very great practical value in controlling 

 and directing breeding experiments with animals and plants. 

 Mendel's law is somewhat complex and difficult to understand, 

 but the essential features of it -are as follows : 



Unit characters. It is an assumption of Mendel's law that, 

 in many cases at least, different characters of animals are unit 

 characters. By this term is meant that those characteristics 

 are handed to the offspring as single units, which are inherited 

 by the offspring in toto or not inherited at all. They cannot be 

 halved or reduced in total characteristics. In other words, if 

 the offspring inherits one of these unit characters, it inherits it in 

 full. Even though the offspring should come from two parents, 

 one of whom possessed the character in question, while the 

 other did not, the offspring would either inherit it as a whole 



