156 THE EVOLUTION OF MAN. 



tion of the germ is so great, that Baer expressed the most 

 general result of his researches in the following proposition : 

 " The history of the evolution of an individual is the his- 

 tory of the growth of individuality in every relation." 

 Whenever a unit, an individual, develops in nature, growth 

 is the first condition. This is equally true of inorganic 

 (inanimate) and of organic (animate) natural bodies. In 

 the former, in minerals, growth is often the only function 

 of evolution. Growth is, therefore, especially interesting, 

 because both in the inorganic individual, the crystal, and 

 in the simplest organic individual, it is the necessary pre- 

 liminary to all further evolution. Growth, the addition 

 of homogeneous body-substance, is absolutely universal. 

 The inorganic crystal grows by absorbing homogeneous 

 matter from the surrounding fluid medium, which then 

 passes from a fluid into a solid condition. Similarly, the 

 cell, the simplest organic individual, grows by attracting to 

 itself particles from the surrounding medium, which is 

 usually fluid, and by then transforming these particles into 

 a semi-fluid, and more or less homogeneous condition 

 (assimilation). The only difference between the growth of 

 the crystal, and that of the simplest organic individual, the 

 cell, is that the former adds the new substance externally, 

 while the latter absorbs it internally. This essential differ- 

 ence depends on the different conditions of density, or of 

 aggregation, of the two different groups of bodies. The 

 inorganic bodies may be either in a solid, fluid, or gaseous 

 condition. They grow by apposition. Organic bodies, on 

 the contrary, are in the fourth, the soft or semi-fluid con- 

 dition of aggregation. They grow by intussusception. 



Each individual or trophic growth is, however, only the 



