Modification and Variation. 313 



most strikingly exhibited in the structure of the spongy 

 substance of the long bones in the higher vertebrates. 

 This substance is arranged on a similar mechanical 

 principle to that of arched structures in general: it is 

 composed of numerous fine bony plates, so arranged as 

 to withstand the greatest amount of tension and pressure, 

 and to give the utmost firmness with a minimum expendi- 

 ture of material. But the direction, position, and strength 

 of these bony plates are by no means congenital or deter- 

 mined in advance : they depend on circumstances. If the 

 bone is broken and heals out of the straight, the plates of 

 the spongy tissue become rearranged so as to lie in the new 

 direction of greatest tension and pressure : thus they can 

 adapt themselves to changed circumstances." 



Then, after referring to the explanation by Wilhelm 

 Eoux, of the cause of these wonderfully fine adaptations 

 by applying the principle of selection to the parts of the 

 organism in which, it is assumed, there is a struggle for 

 existence among each other, Prof. Weismann proceeds 

 to show * " that it is not the particular adaptive struc- 

 tures themselves that are transmitted, but only the quality 

 of the material from which intra-selection forms these 

 structures anew in each individual life. ... It is not the 

 particular spongy plates which are transmitted, but a cell- 

 mass, that from the germ onwards so reacts to tension and 

 pressure that the spongy structure necessarily results." In 

 other words, it is not the more or less definite congenital 

 adaptation that is handed on through heredity, but an 

 innate plasticity which renders possible adaptive modifica- 

 tion in the individual. 



This innate plasticity is undoubtedly of great ad- 

 vantage in race progress. The adapted organism will 

 escape elimination in the life struggle ; and it matters not 

 * Eomanes Lecture, p. 1 5. 



