SPAWNING BEHAVIOR IN FUNDULUS HETEROCLITUS. 345 



phase of color intensification that is simply dependent on an in- 

 crease of pigment ; the blue gleam that appears only during active 

 spawning must be the direct result of sexual excitement, for it 

 fades almost immediately when the males are removed from the 

 sexual environment. I am inclined to look upon this gleam as 

 a sort of flush such as might suffuse the human body under ex- 

 citement. 



Contact organs may be considered as excrescences produced 

 by the excess vitality of the male and specialized for an especial 

 function. As suggested in the introduction of this paper, it is 

 believed that both structure and function appeared in response to 

 a heightened metabolism. After their appearance the structures 

 were modified by use or function. 



It is interesting to endeavor to trace the origin and develop- 

 ment of the habit of intromission that seems to prevail among 

 about half of the Pceciliidae. This habit is invariably associated 

 with an increase in the length of the anterior rays of the anal 

 fin and the modification of these rays in various ways into an 

 organ of intromission. The first step in the process was doubt- 

 less a mere elongation of the whole anal fin, as seen in Fundulus 

 diapJianns. The next step was probably a more rapid growth 

 of the anterior rays of the fin, such as we see in Fnndnlus hetcro- 

 clitns and to a slightly more marked extent in Fnndnlus majalis. 

 The function of this elongation in these species of Fundulus is 

 partially to give the male a greater reach and partly to fan the 

 sperm toward the extruded eggs. It is only a few steps farther 

 in the same direction for the anal fin to assist the sperm to enter 

 the oviduct of the female when internal fertilization becomes 

 necessary. 



