202 CALIFORNIA ACADEMY OF SCIENCES. 



nition marks by which individuals are enabled to 

 mutually avoid one another, as, for instance, the dis- 

 tinctions of species by characteristic marks, by which 

 infertile crosses are prevented. These two classes may 

 be termed respectively directive and discriminative 

 recognition marks. A third class includes such marks 

 as are of use in the intercourse of sexes, the care of 

 young, or in the darkness of holes or burrows. For 

 these the term socialistic marks may be used. 



In the pajjer in Zoe cited above I found difficulty in 

 explaining these recognition marks in harmony with the 

 survival of the fittest, for the individuals possessing 

 these marks are not thereby benefited but rather 

 benefit others. In the introduction to this paper (see 

 ante p. 108), I have indicated the manner in which these 

 might be explained through the necessity for the sur- 

 vival of the family. 



Directive marks may be of three sorts: those which 

 enable stragglers at a distance to regain the flock — espe- 

 cially well illustrated by a flock of moving sandpipers, 

 which as they wheel about now flash into sight as a flut- 

 tering mass of shining white and then disappear, like a 

 flash light which throws its rays now upon one point of 

 the compass and now upon another — those which enable 

 a bird to follow its companion or companions through 

 dense underbrush, as for instance some of the towhees, 

 (Pipilo), which are so conspicuously marked with black 

 and white, and which always frequent the densest tangle 

 of the bushes — and those which enable migrating birds 

 to follow their leaders. As migrations are so largely 

 conducted by night, call notes and the general manner of 

 flying are probably more relied on for purposes of recog- 

 nition than color marks, although I am inclined to 

 tljink that the latter may at least in some instances also 

 be called into play. Todd has suggested a number of 



