1 62 BIOLOGICAL LECTURES. 



an immune species, which may belong to a different order or 

 family. Bates, in his classic paper upon the " Heliconidas of 

 the Amazon Valley," first called attention to this remarkable 

 fact and designated it very aptly as "mimicry." A still more 

 remarkable mimicry is seen where one immune species resem- 

 bles another immune species. For many years the explanation 

 of this curious form of mimicry remained a puzzle to naturalists, 

 until Fritz Miiller in 1878 showed that, if it be true that young 

 insectivorous birds have to find out by experiment what insects 

 are good to eat and what ones are not, and if they remember 

 to avoid evil-tasting species after once having tried them, then 

 it becomes of advantage to both species of butterflies if one dis- 

 tasteful insect comes to resemble another. But we have not time 

 to discuss this interesting topic. Suffice it to say that a series 

 of careful and elaborate experiments, carried out by Mr. Frank 

 Finn upon insectivorous birds of India, demonstrate that birds 

 do remember to avoid retasting a noxious insect after once 

 having had experience with it, and that most birds soon learn 

 to beware of brightly colored butterflies which have a disa- 

 greeable taste. 



In many species of moths and butterflies the males are quite 

 different in color from the females of the same species. Dar- 

 win sought to explain this upon the hypothesis that the females 

 exercised selection in choosing their mates, only males of a cer- 

 tain shade of color being popular, and that thus the males came 

 to depart more or less widely from the females in coloration. 

 With this hypothesis in view I have very recently carried out 

 a series of experiments upon our common Callosamia prome- 

 thea. In this moth the female is ferruginous, while the male 

 is black. As is well known, the males of this species display 

 a remarkable aptitude in seeking out the females, and they 

 always fly up against the wind to the place where the female 

 is resting. My experiments show that the males are not at all 

 attracted by the mere sight of the female, but that they are 

 attracted by some substance which emanates from the abdomen 

 of the female. Females about six hours old are not as attract- 

 ive as are females thirty hours old, and dead or dying females 

 are not visited by males. Also, if the female be deprived of 



