246 THE EVOLUTION THEORY 



Struggle of the Parts, and I subsequently defined the process as 

 histonal or tissue selection. 



Let us first take an example. The anatomist Hermann Meyer 

 showed in 1869 that the so-called 'spongiosa,'-that is, the bony tissue 

 of spongy structure within the terminal portions of the long bones 

 in Man and Mammals, has a minute structure conspicuously well 

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

 precisely in the direction of the strongest strain or pressure which is 

 exerted upon the bone 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 had been entrusted with the task of making 

 a complicated system of arches with the greatest possible carrying 

 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-diiferentiation, 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 largest 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 lamellae 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, which have the power of differentiating into one or another kind 

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



