XVI ARE ACQUIRED CHARACTERS INHERITED? 323 



of the skin for the origin of which we can assign no cause. 

 Many other secondary sexual appendages of birds are 

 equally inexplicable in their beginnings, such as the long 

 feathers springing from the sides of the head in the six- 

 plumed bird of paradise, and the singular pair of long 

 white feathers growing from among the upper primary 

 wing-coverts of Semio'ptera vjallacei, to which I believe 

 there is nothing similar in the whole class of birds. These 

 various cases of dermal appendages are sufficient to 

 indicate that variations of this kind are continually 

 occurring, which, whenever useful, have been seized upon 

 and increased by natural selection, since any such varia- 

 tions appearing among our domesticated animals are found 

 to be strictly inherited. 



Evolution of Teeth. 



The American naturalists lay much stress on the 

 evolution of the teeth of mammals in complete palseonto- 

 logical series, alleging that the successive modifications of 

 the cusps conform strictly to lines of use and disuse, and 

 that they are therefore produced by use and disuse. To 

 this there are two distinct replies, either of which seems 

 to me sufficient. In the first place, in such vitally 

 important organs as the teeth of mammals, natural selection 

 will necessarily keep them on these lines, because use 

 implies utility, and disuse, inutility, and utilities necessarily 

 survive. If, then, variations occur in the forms of the 

 cusps — and they certainly do — natural selection will modify 

 them along these lines of utility ; and it will be absolutely 

 impossible, from a study of the series of fossil forms, to 

 prove that they have been directly modified by use and 

 that the modifications have been inherited, and that they 

 are not the result of normal variations accumulated by 

 survival of the fittest. The second reply is that of 

 Professor Poulton, who points out that the form of the 

 tooth is fixed before it cuts the gum and that use only 

 wears the cusps away. It is therefore difficult to see how 

 such use in the parent can determine any definite varia- 

 tion in the teeth of the next generation. 



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