96 Natural TJicolcgy. 



forth to deposit its egg in the limb of the oak, that 

 in due time, after her own death, her young will find 

 their resting-place in the earth ? She obeys a blind 

 impulse. The limb is pierced and the egg deposited 

 She has by instinct made use of both structure and 

 function of organs, and thus completed her adapta- 

 tion to the world in preserving the species. This is 

 only one instance from a great number in which the 

 parent never sees its young, and never can see them, 

 and yet provides completely for them. 



And the young cicadas that hatch from those eggs 

 after their parents have perished and disappeared, 

 find their way to the earth, and there, under the 

 guidance of this instinct, provide for themselves ; 

 Nature taking care that the development of all the 

 thousands shall progress so uniformly, that when 

 seventeen years have passed away, on a given 

 day they are all ready to come forth for their few 

 gala-days of life. As they come forth only at the 

 end of seventeen years, and then live only a few 

 days, it is necessary for the preservation of the 

 species that they should all come forth at the same 

 time. They are like ten thousand chronometers all 

 set to run seventeen years, and all the changes in 

 form are accompanied by change in instinct, so that 

 when the years have revolved, structure and instinct 

 have completed their work, and the ten thousand 

 chronometers are all found in perfect adjustment. 



The tent-moth also lays her eggs upon the apple- 

 twig, closely packed and varnished to protect them 

 till the warmth of spring wakes the young to life, 



