SPAWNING BEHAVIOR IX FUNDULUS HETEROCLITUS. 343 



gin of this adaptation and the way in which it is thought to have 

 given rise to the habit of intromission will be discussed later. 



The differences in shape of certain fins may, with equal cer- 

 tainty, be said to be adaptive. The backwardly directed prolonga- 

 tions seen on the anal fins of all the species of Fnnduhis exam- 

 ined are evidently to give the male a greater reach in his effort to 

 clasp firmly the body of the female. The shortness and softness 

 of the dorsal fin of the female is of advantage in that it is thus 

 less in the way. 



Differences in color pattern can only be explained as sex rec- 

 ognition marks. It can readily be seen, in the light of the obser- 

 vations on the spawning behavior of these fish, that it must be 

 possible for the males to recognize the females at once and at a 

 distance. Some distinctive character that would appeal to the 

 visual sense is required. In the course of evolution those females 

 that could readily be recognized as females would be the ones 

 most frequently mated with and they would be the most likely 

 to transmit this variation to their descendants. There seem to 

 be at least two means of acquiring a distinctive appearance that 

 would serve to mark off the females from the males. One means 

 is to lose the common racial marking more or less completely, 

 and thus to acquire a sort of secondary solid coloration. This 

 method can be seen in two stages of development in two species 

 of Fundulns, v\z.,F. diaplianns and heteroclitus. The former pre- 

 sents a condition in which the bars of pigment of the female are 

 simply narrowed markedly, while the latter shows their reduc- 

 tion to the merest suggestion of a cross-banded pattern. The 

 second means of acquiring a distinctive female marking is exem- 

 plified by F. niajalis, in which the characteristic banded pattern 

 of the species, which is possessed by the young of both species, 

 is, during ontogeny, gradually converted into a longitudinal strip- 

 ing of a most pronounced order. The stages in the process of 

 change from a transverse to a longitudinal pattern have been 

 described in another place. 



Attention has been called to the sequence of changes from the 

 young to the adult color pattern in both male and female of Fun- 

 dulus inajalis. In the first place there seems to be a very deep- 

 seated law of antero-posterior development. In the second place 



