188 THE INDUCTIONS OF BIOLOGY. 



formed and combined — if not in datail yet in their general 

 characters. This, however, is an error. There are certain 

 truths of structure higher in generality than any which can 

 be alleged of organs. We shall see this if we compare organs 

 with one another. 



Here is a finger stiffened by its small bones and yet made 

 flexible by the uniting joints. There is a femur which helps 

 its fellow to support the weight of the body ; and there again 

 is a rib which, along with others, forms a protective box 

 for certain of the viscera. Dissection reveals a set of muscles 

 serving to straighten and bend the fingers, certain other 

 muscles that move the legs, and some inconspicuous muscles 

 which, contracting every two or three seconds, slightly raise 

 the ribs and aid in inflating the lungs. That is to say, 

 fingers, legs, and chest possess certain structures in common. 

 There is in each case a dense substance capable of- resisting 

 stress and a contractile substance capable of moving the 

 dense substance to which it is attached. Hence, then, we 

 have first to give an account of these and other chief ele- 

 ments which, variously joined together, form the different 

 organs : we have to observe the general characters of tissues. 



On going back to the time when the organism begins with 

 a single cell, then becomes a spherical cluster of cells, and 

 then exhibits differences in the modes of aggregation of these 

 cells, the first conspicuous rise of structure (limiting our- 

 selves to animals) is the formation of three layers. Of these 

 the first is, at the outset and always, the superficial layer in 

 direct contact with the environment. The second, being origi- 

 nally a part of the first, is also in primitive types in con- 

 tact with the environment, but, being presently intro- 

 verted, forms the rudiment of the food-cavity; or, otherwise 

 arising in higher types, is in contact with the yelk or food 

 provided by the parent. And the third, presently formed 

 between these two, consists at the outset of cells derived 

 from them imbedded in an intercellular substance of jelly- 

 like consistence. Hence originate the great groups classed 

 as epithelium-tissue, connective tissue (including osseous tis- 



