VARIATION 21 



cited as an illustration of harmful variations. The 

 amount of closure of the opening from the intestine into 

 the vermiform appendix in man is an example of a 

 harmful variation, since the larger the opening, the 

 greater is the liability to appendicitis. 



d. With respect to their direction in evolution varia- 

 tions may be either definite (orthogenetic) or indefinite 

 (fortuitous). 



Fortuitous or chance variations in all possible direc- 

 tions furnish the repertory of opportunity, according to 

 Darwin, from which natural selection picks out those 

 best adapted to survive in the struggle for existence. 



Paleontology furnishes numerous instances of the 

 former category, such as the series of variations from 

 a pentadactyl ancestor, all apparently tending in one 

 direction, which have culminated in the one-toed horse. 

 The fact that the paleontologist deals historically with 

 a completed phylogenetic series in which the side lines 

 lack prominence, while the successful line stands out 

 with distinctness, makes it easy for him to view succes- 

 sive variations as orthogenetic, that is, as definitely di- 

 rected in one course either through intrinsic (Nageli) or 

 extrinsic (Eimer) causes. 



Just as sometimes the individuals of an apparently 

 continuous series, as shown in a museum collection of 

 similar insects, may be of very diverse geographical 

 origin, so genetics is not primarily concerned with re- 

 semblances from generation to generation but rather 

 with origins and continuity. 



e. With respect to their source, variations may be 

 somatic or germinal. Somatic, or body variations, 



