320 ELEMENTS OF BIOLOGY 



mitted from generation to generation. For convenience we will 

 designate such differences as variations. New variations which ap- 

 pear within the species that are inheritable are spoken of as muta- 

 tions. It is with this group that the present discussion will deal. 

 (3) The third group of characters are those caused by environ- 

 mental factors. In cats such differences are largely confined to differ- 

 ences in heaviness of coat, mutilations as the result of injuries, etc. 

 In Man these characters include training and education. All these 

 are not of themselves inherent in the germ cells but are acquired 

 by the individual through contact with its environment. To dis- 

 tinguish such features from variations we may designate them as 

 modifications, or acquired characters. 



Mendel. The fundamental laws now known to govern the in- 

 heritance of variations within the species were first discovered by 

 an Austrian monk, Gregor Mendel, and published in a little known 

 journal in 1863. Mendel's work was practically unknown until the 

 close of the nineteenth century, when, during the increased impetus 

 given to the study of inheritance by Darwin, Weismann, and others, 

 DeVries, a Dutch investigator, came upon Mendel's paper and pre- 

 sented it anew to the scientific world. The current century has seen 

 the development of the principles set down by Mendel as results 

 of his breeding experiments, and the growth of Mendelism as a 

 science of far-reaching academic and economic importance. 



Alternative Characters. While not the first to use the method 

 of direct experiment, nor the first to analyze his results by means 

 of mathematics, Mendel's clear-cut methods and the success which 

 he attained in revealing important principles of vital phenomena 

 gave a strong impetus to the employment of similar methods in 

 the study of a wide variety of problems. He recognized the neces- 

 sity of confining attention to single facts among the maze of inher- 

 itable characters, and of tracing in detail the events surrounding 

 these facts. His breeding materials were peas and he confined his 

 attention to seven characters that varied between parent and off- 



