156 



HABIT AND INTELLIGENCE. 



[chap. 



and struc- 

 tural. 



CeUuIar 

 tissue. 



Differen- 

 tiation of 

 tissues. 



Growth 

 and deve- 

 lopment 



antago- 

 nistic. 



Leaves and 

 flowers. 



bile from the blood, another milk, and so on ; for these 

 various results are affected by glands, all of which consist 

 of substances of the same chemical constitution ; and con- 

 sequently would all act alike, if their action were merely 

 a chemical one. 



The next vegetative function consists in the arrangement 

 of the organic compounds so as to form tissue. As already 

 stated, the simplest tissues are cellular, and cell-formation 

 consists in the separation, or differentiation, of the primary 

 structureless germinal matter into consolidated substance, 

 which forms the outside of the cell; and soft, almost 

 fluid, substance which constitutes the cell-contents, and, 

 at least in the simplest organisms, retains the properties of 

 germinal matter. Many organisms, as for instance the 

 lowest Algae, consist of but a single cell, which propagates 

 by spontaneous division. But in others the cells, after 

 dividing, do not separate, but remain together ; and thus 

 cellular tissue is formed. The unicellular and multicellular 

 forms of Algee graduate into each other, and the Algge in 

 general consist of a mass of cellular tissue, with little 

 further differentiation. 



Cells, in the various parts of various organisms, undergo 

 endless modifications, both in form, and by acquiiing the 

 power, as stated above, of separating different substances 

 from the sap or blood. Accordingly, the next differentia- 

 tion consists in the acquisition of different characters by 

 different masses of cells, so as to form different tissues : as, 

 for instance, soft leafy substance and hard woody fibre, in 

 plants ; and muscle, nerve, and bone, in animals. 



The formation of tissues constitutes growth, and the 

 differentiation of tissues the one from the other constitutes 

 development. It is important to observe that growth and 

 development are not the same thing ; they do not imply 

 each other, and do not necessarily go on together — indeed, 

 there is frequently an antagonism between them; rapid 

 growth and rapid development appear, at least in certain 

 cases, to be incompatible. Tlius, flowers are more highly 

 developed than leaf-bearing branches ; and flower-bearing 

 branches are always found to have lost something of the 



