VI. 
THE PHYSICAL BASIS OF HEREDITY. 
By FRANK MAcE McFaRLanD. 
ALt living organisms, animals as well as plants, are 
built up of certain elementary parts or units termed 
cells. No matter how widely divergent 
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 (1838-39) 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. 
The cell theory. 
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