364 Comparative Anatomy — Its History, Aim, and Method 



Fig. 290. Continuity of the germ-plasm, the basis for homology. 



Each animal outlined in the diagram must be regarded as representing both 

 male and female parent. The heaviest lines represent the main line of germ-plasm 

 passing from one adult generation to the next via the fertilized egg and the develop- 

 ing embryo. In the single ( o" + 9 ) individual of Generation I the several fine lines 

 suggest the distribution of embryonic products of the germ-plasm to form the 

 several parts of the animal. G is the gonad. The organ chosen for special considera- 

 tion is the pectoral fin, A. The complex of germinal factors determining the develop- 

 ment of A is represented by a. 



Along the left one of the two lines of germ-plasm proceeding from Generation I, 

 no change occurs in a through Generations II and III and thence onward through 

 an indefinitely great number of generations to the Nth generation (N) in which A is 

 still a fish fin. 



Along the right one of the two lines of germ-plasm, a persists unchanged into 

 Generation III. But in some of the members of III it undergoes change and emerges 

 as ax and the adult organ in succeeding generations becomes A x . In the course of an 

 indefinitely great number of generations (the broken lines indicating their omission 

 from the diagram), additional modifications are incurred. The ax becomes axr. 

 Meanwhile correlated modifications occur in the germinal factors which determine 

 other organs, with the result that, at the Nth generation, A xr is a foreleg of a reptile. 

 Along another line of descent proceeding from Generation III, axr undergoes still 

 further modification and becomes axrb which produces an organ A xrb . Meanwhile, 

 correlated changes occur in the other organs, and eventually A irh appears as a bird's 

 wing. 



Organs A, A x , A XT , and A irb are homologous because they all possess the basic 

 a which has come down to them via the germ-plasm from the common ancestor, 

 Generation I. 



