168 DARWINISM TO-DAY. 



hair, etc., etc., depends on the various stages of development 

 reached by the reproductive organs. Now these secondary 

 organs or modifications thus produced through the influence 

 of other organs may be for a while slight and indifferent 

 in character, but yet safely maintained. When they reach 

 a stage of utility or of positive disadvantage they will then 

 be further perfected, or on the other hand be extinguished, 

 by selection. 



The principle of the change of function (Fnnctious- 



wechsel) first elaborated by Dohrn, 7 is also called on by 



Dohrn's rin ^ ate to P^ a X an important part in explaining 



cipie of change how an organ of considerable specialisation can 



off-unction. be shown to haye been developed by selection, 



although the function it is now performing seems to be one 

 that could have been useful only in a perfected state and 

 hence could not have made the organ so constantly ad- 

 vantageous in all the slow and gradual stages of its evolution 

 as to be of selective value in its beginning stages. Dohrn's 

 principle is stated as follows : "An organ can, in its service 

 of a certain definite useful function, be developed by natural 

 selection to a certain stage. Simultaneously a second func- 

 tion (Functionserweiterung) can have developed, due to 

 some special peculiarity or condition of the position, struc- 

 ture, or capacity for movement, which may have a value in 

 another direction from that of the first function. Thus 

 the appendages of crabs serve often special functions in rela- 

 tion to respiration, copulation, and care of the eggs or 

 young, while their original locomotory function may still 

 be maintained or may be more and more surrendered in 

 favour of the new functions." Numerous other specific ex- 

 amples are obvious enough to any student of biology. Now 

 the new functions in many cases become the more import- 

 ant so that there in time results a complete change of func- 

 tion which wholly alters the physiological character of the 

 organ, and in many cases it is difficult to see (if one does 



