CHAPTER VII 



THE MECHANISM OF DEVELOPMENT AND DIFFERENTIATION 



Before specific inquiries can be profitably made into the causes 

 of variation it is necessary to become fairly familiar with what 

 is known of the essential constitution of living matter and of its 

 manner of growth and differentiation. 



After attention has been bestowed for a time upon these con- 

 siderations, it will be evident to the student that here, in the 

 inner workings of living matter, are fundamental causes of pro- 

 found variations, even in protoplasm seemingly the most stable. 



SECTION I PROTOPLASM THE PHYSICAL BASIS 

 OF LIFE 



Protoplasm is a general name for all matter that is endowed 

 with life, but the student must never forget that the biologist, 

 like the chemist, is dealing with matter composed of well-known 

 chemical elements united in definite proportions. The known 

 differences between living and non-living matter are, for our 

 purposes, the following : 



1. Living matter is endowed with a mysterious force called 

 life. 



2. Matter so endowed has a much more complicated chemical 

 composition than has non-living matter, or than can be main- 

 tained after the life principle has departed. Living matter at 

 death, therefore, breaks up (or down) into ordinary chemical 

 compounds. The true constitution of living matter cannot, 

 therefore, be determined by any known methods of analysis, 

 which reveal only the elements involved but not their exact 

 relations during life. 



3. Matter endowed with life is able to appropriate to itself 

 other outlying matter and to increase its bulk through growth. 



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