THE MECHANISM OF DEVELOPMENT 143 



4. This growth is not of bulk merely, but it is attended by 

 differentiation, so that one part is distinctly different from 

 another. 



5. As growth proceeds bits of this bulk are thrown off, each 

 of which constitutes a new individual capable of independent 

 existence, reproduction. 



6. The new individual is substantially, but never exactly, 

 like the one from which it arose, and here lie the chief mys- 

 teries of breeding. In reproduction there are no duplicates. 



Nothing approaches this in the inorganic world save crystal- 

 lization. Crystals add matter to their bulk and thus may be 

 said to grow. Moreover, the matter is added in an orderly 

 manner, resulting in a kind of definite structure with exact 

 angles always the same, but nothing like differentiation exists. 

 One part of the crystal is like another ; it has no power of 

 reproduction and is possessed of no force comparable with life. 



The student should early learn that the field of biology is 

 distinct, but he should also fully realize that it lies within and 

 not outside the range of chemistry, and that living matter is not 

 freed from its ordinary affinities by reason of its association with 

 life, but on the contrary it continues as before to be subject to 

 the ordinary physical and chemical relations of matter generally. 

 If he can do this, he will simplify many of his difficulties. 



SECTION II THE CELL THE UNIT OF STRUCTURE 



If a bit of liver, bone, wood, or any other form of plant or 

 animal tissue be examined under the microscope, it will be found 

 to possess a definite structure, and to consist of a large number 

 of separate divisions, each filled with a gelatinous mass called 

 protoplasm. These separate divisions or cells are apparently 

 alike throughout the substance of any particular tissue, as 

 the liver, but they differ greatly in different tissues of the 

 same body (bone, muscle, brain). Biologists have been unwilling 

 to consider the individual as the unit, because he is too large 

 and his structure and activities are too complicated. They have, 

 therefore, chosen to regard the individual as a colony of many 

 and variously differentiated cells. 



