58 AN INTRODUCTION TO ZOOLOGY. 



in which an organ, or the rudiment of an organ, sub- 

 sequently disappears gradually, or, as it is expressed 

 in technical language, becomes atrophied. It has per- 

 haps never attained suflBcient perfection to be of any 

 use to the owner, or it may be of such a nature as to 

 be of no use unless the animal were adult and free. 

 What reason can be assigned for the existence of such 

 a structure, except that it is a feature inherited from 

 an ancestor which, in its adult stage, possessed the 

 complete structure and made use of it ? 



Then, again, there are cases in which bones, which 

 in a lower type of animal are distinct, appear as dis- 

 tinct centres of ossification in the embryo of a type 

 which has them fused into one in the adult. An 

 instance of this appears in the coraeoid process of 

 the mammalian shoulder - bone. This bone, like 

 other bones, at first consists of cartilage ; and the 

 bone-making process begins and spreads outward 

 from an internal centre, technically called the nucleus. 

 The scapula has, besides its chief nucleus, a little 

 separate one, which eventually forms the coraeoid pro- 

 cess. Now, in the Ornitliodelphia or Monotremes, 

 the lowest members of the class mammalia (see 

 p. 280), the coraeoid exists as a separate bone, 

 even in the adult. Instances of this kind are very 

 suggestive. 



It is not the force of any one instance, but the 

 cumulative evidence of the immense number of in- 

 stances, which are intelligible by the aid of a certain 

 theory and by that theory alone, that has led em- 



