THE SCOPE OF BIOLOGY. 101 



ation of functions by structures, and the determination of 

 structures by functions. 



As displayed in individual organisms, the action of struc- 

 tures on functions is to be studied, not only in the broad and 

 familiar fact that the general kind of life an organism leads 

 is necessitated by the main characters of its organization, but 

 in the more special and less conspicuous fact, that between 

 members of the same species minor differences of structure lead 

 to minor differences of power to perform certain kinds of action, 

 and of tendency to perform such kinds of action. Con- 



versely, under the re-actions of function on structure as 

 displayed in individual organisms, come the facts showing 

 that functions, when fulfilled to their normal extents, main- 

 tain integrity of structure in their respective organs ; and 

 that within certain limits, the increase of functions is followed 

 by such structural changes in their respective organs, as en- 

 ables the organs to discharge better their extra functions. 



Inquiry into the action of structure on function as dis- 

 played in successions of organisms, introduces us to such 

 phenomena as Mr. Darwin's " Origin of Species " deals with. 

 In this category come all proofs of the general truth, that 

 when an individual is enabled by a certain structural pecu- 

 liarity, to perform better than others of its species some 

 advantageous action ; and when it bequeaths more or less of 

 its structural peculiarity to descendants, among whom those 

 which have it most markedly, are best able to thrive and 

 propagate ; there arises through this continuous action of 

 structure on function, a visibly modified type of structure, 

 having a more or less distinct function. In the cor- 



relative class of facts, which come under the category of re- 

 actions of function on structure as exhibited in successions of 

 organisms, are to be placed all those modifications of struc- 

 ture which arise in races, when changes of conditions entail 

 changes in the balance of their functions. Here is to be 

 studied the way in which altered function externally necessi- 



