PROGRESSIVE HEREDITY. I71 



reversion to the three-toed historical ancestors of the 

 present genus. These vouchers are sufficient. 



All the phenomena of artificial breeding, as well as 

 natural selection, serve to show that not only the cha- 

 racters descended from past ages, but also those subse- 

 quently and most recently acquired, may be transmitted 

 to posterity. This is progressive heredity. Without 

 it, improvement and progress would be impossible ; and 

 its own possibility is the direct result of the nature of 

 reproduction. The newer a useful modification, the less 

 has it hitherto been able to place itself in correlation 

 with the entire organism, the less is the reproductive 

 system as yet affected by it ; the more uncertain and 

 fluctuating, therefore, is the transfer by propagation ; 

 breeding, or natural selection, is requisite to convert the 

 potentiality of progress into a fact, and gradually to 

 enrol this fact among the conservative inheritances. 



Progressive heredity is naturally more complex where 

 the sexes are separate, where sexual selection asserts 

 its rights, and the advantages of one sex are fostered 

 by the taste of the other, and are then either trans- 

 ferred exclusively to the sex benefited by its secondary 

 characters, or turned to the profit of the whole species. 

 As a rule, the males are endowed with these advantages, 

 and have transmitted them incompletely to the females. 

 We will explain ourselves by a single example. In the 

 order of insects termed Orthoptera (or straight-winged), 

 the males, by rubbing their wing-covers together, or by 

 stroking them with the lower portion of their hind legs, 

 are able to make a music attractive to the females. 

 Von Graber, a distinguished modern entomologist, has 

 shown ^^ that the teeth of the stridulating instruments of 



