VI. 



THE PHYSICAL BASIS OF HEREDITY. 

 BY FRANK MACE MCFARLAND. 



ALL living organisms, animals as well as plants, are 

 built up of certain elementary parts or units termed 



cells. No matter how widely divergent 

 The cell theory. , * . 



in external appearance or habitat they 



may be, the elephant and the lily, the sponge and the 

 palm, are each aggregations of structural units, funda- 

 mentally alike, and no form of animal or plant life is 

 known to exist which does not conform to this general 

 law. To the studies of Schleiden upon plants and of 

 Schwann upon animals (iS^S-'^) we owe the foundation 

 of the "cell theory," more precisely formulated by Max 

 Schultze in 1861. Since the time of these pioneer 

 studies upon the cell, investigation has been carried on 

 by a constantly increasing number of students with 

 methods and instruments steadily improved in their 

 efficiency, and the accumulated results already throw a 

 wealth of light upon some of the most abstruse prob- 

 lems of biology. Yet the most enthusiastic and san- 

 guine of these workers will not assert that we have 

 advanced further than the threshold of this domain in 

 which are concealed the answers to the questions as to 

 the ultimate structure of living matter and even to the 

 very nature of life itself. 



