246 THE EVOLUTION THEORY 



Stnicfgle of the Partx, and I subsequently defined the process as 

 histonal or tissue selection. 



Let us first take an example. The anatomist Hermann Mej^er 

 showed in i<S69 that the so-called 'spongiosa,' that is, the bony tissue 

 of spong3' structure within the terminal portions of the long bones 

 in Man and Mannnals, has a minute structure conspicuously well 

 adapted to its office. The thin bone lamellas of this ' spongiosa ' lie 

 precisel}^ in the direction of the strongest strain or pressure wdiich is 

 exerted upon the bpne at the particular area. Arch-like in form, they 

 are kept apart by means of buttresses, and no architect could have 

 done better if he liad been entrusted with the task of making 

 a complicated system of arches with the greatest possible carr^'ing 

 and resisting power combined with the greatest possible economy of 

 material. 



This well-adapted structure is now interpreted through the 

 Struggle of the Parts as a self -differentiation, for if there be in the 

 rudiments or primordia of the bone differently endowed elements^, that 

 is, cells which respond in diverse ways to different stimuli, these must 

 arrange themselves locally, owing to the struggle for space and food, 

 in a manner corresponding to the distribution of the different stimuli 

 in the bone. The laro-est amount of bone substance will be formed in 

 the directions of the strongest strain and the greatest pressure, because 

 the bone-forming cells are excited by this, their functional stimulus, 

 to growth and multiplication. Thus the buttress and arch structure 

 comes about, and between the delicate bone lamella spaces will remain 

 free, and these, being relieved from the burden of strain and pressure 

 by the aforesaid bony lamellae, will offer suitable conditions of life to 

 cells with other functional properties, such as connective tissue cells 

 or vascular cells. 



The structure of the bone spongiosa is not everywhere the same, 

 and it is demonstrably related with precision to the conditions of 

 strain and pressure at each particular region. Thus, just below the 

 soft cartilaginous covering of the joints there are no long pillars with 

 short arches, but only rounded meshes, because the pressure is here 

 almost equally strong from all sides. The long parallel pillars only 

 occur further down in the bone, and they lie in two directions which 

 intersect each other obliquely, corresponding to the two main 

 directions of pressure. But it is only under the functional stimulus 

 of pressure that the bone-forming cells have an advantage over the 



^ I do not here enter into the question whether we have not in this case to do with 

 similar elements, Avhich have the power of diffei-entiating into one or another kind 

 of cell according to the nature of the external stimuli by which they are influenced. 



