134 INTRODUCTION TO ZOOLOGY. 



carried by the blood to the tissues where it is to be 

 used. It really seems as if the introduction of the 

 blood as a carrier, or "middleman," between the oxy- 

 gen and the tissues where it is to be used is a rather 

 clumsy contrivance. 



"We have seen the third eyelid, or nictitating 

 membrane, in the pigeon and in the turtle. It is a 

 very useful device to those animals, enabling them to 

 see, in some degree at least, while the eye is still pro- 

 tected. We have seen the same organ, modified, in 

 the eye of a rabbit. The same structure is very 

 apparent in the eye of a dog, or in the eye of a horse, 

 and to a less degree in the human eye. In the mam- 

 mals generally, however, the structure is not at all 

 useful. It is an organ such as is called a vestigial 

 organ. Vertebrates generally, and mammals in par- 

 ticular, show many examples of vestigial organs. In 

 the human body, that part of the intestinal ceecum 

 which is called the vermiform appendix is not only 

 useless, but is a positive source of danger, being sub- 

 ject to a disease called appendicitis, which frequently 

 causes death. 



A rabbit or a horse can move its ears. The mus- 

 cle to move the ears exists in every human being, but 

 only a very few people can move their ears. The 

 muscle is vestigial. 



The only exjDlanation that is at all probable for 

 vestigial organs is that sometime in the history of the 

 species they have been useful, or, at least, functional. 

 But in consequence of changed conditions they have 

 lost their usefulness, and as a result, either directly 

 or indirectly, they have lost their functions. 



The farther along the j^ath of development an 

 animal has proceeded, and the more highly special- 

 ized it has become, the greater the number of vestigial 

 organs we may naturally expect it to show. 



