248 THE EVOLUTION THEORY 



■would crowd out other variations at that spot, just as the l)ody 

 and its individual parts may be said to have taken their architectural 

 form in exact response to the demands made upon them by function. 

 In this case, of course, personal selection and histonal selection co- 

 operate, for every improvement in the organization of the fundamental 

 liviuf substance means at the same time a lasting improvement in the 

 whole individual. 



In many-celled organisms, however, we must admit that there is 

 an essential diilerence between personal and histonal selection, inas- 

 much as the latter can give rise to adaptive structural modifications 

 corresponding to the needs of the tissue at the moment, but not to 

 permanent and cumulative changes in the individual elements of the 

 tissue. If a broken bone heals crookedly, the spongy substance within 

 the healed portion does not remain as it was before, for the pillars 

 and arches, which now no longer run in the direction best suited to 

 their function, break up, and a new system of arches is formed, 

 not in line with the earlier one, but adapted to the new conditions 

 of pressure. This is certainly an adaptation through selection, but 

 the elements, that is the cells which form the bone substance in 

 response to strain and pressure, or those which in response to the 

 stimulus of the blood flowing into the spaces form the blood-vessels, 

 or those which being quite freed from one-sided pressure develop into 

 connective tissue, must be presupposed. These kinds of cells must be 

 virtually implied in the germ-rudiment ; they are themselves adapta- 

 tions of the organism, and can therefore only be referred to 2^e^^sonal 

 selection. And this is true of all adaptations of the elements of 

 multicellular organisms, and thus of the cells. Their adaptation 

 according to the principle of division of labour, their differentiation 

 into muscle, nerve, and gland cells can only be referred to natural 

 selection in the Darwin- Wallace sense, and cannot depend upon 

 histonal selection. In the spongy substance of the bone a better 

 bone-cell does not struggle wnth an inferior one and leave behind 

 it by its survival a host of descendants which are, if possible, better 

 than itself; the struggle for existence and for descendants, in this 

 case, is betw^een two kinds of cell wdiich wxre different from the 

 beginning, and of which one has the advantage at one spot, another 

 at another. The case may be compared to that of a flock of nearly 

 allied species of bird, of wdiich one species thrives best in the plains, 

 another among the hills, and a third among the mountain forests, all 

 mingled together in a vast new territory to which they had migrated, 

 and in w^hich all three kinds of conditions were represented. A 

 struggle would arise among the different species, in which in every 



