VARIATION 39 



b. With respect to their duplication variations may 

 be single or multiple. A legless lamb 1 is an ex- 

 ample of a single variation or "sport." Four-leaved 

 clovers, on the contrary, are multiple for the reason 

 that this variation, although not common, neverthe- 

 less occurs frequently. 



c. With respect to their utility variations may be 

 useful, indifferent, or harmful to the organism possess- 

 ing them. Useful variations are of the kind empha- 

 sized by Darwin as being effectively made use of in 

 natural selection. Indifferent variations, on the 

 other hand, are those which apparently do not play 

 an important part in the welfare of their possessor, 

 as, for example, the color of the eyes or of the hair. 

 Finally, the degree of degeneration in certain organs 

 may be cited as an illustration of harmful variations. 

 The amount of closure of the opening from the in- 

 testine into the vermiform appendix in man is an ex- 

 ample 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). 



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 suc- 



1 " A Peculiar Legless Lamb." Stockard. Biol. Bull, xiii, p. 288. 



