EVIDENCE FROM COMPARATIVE ANATOMY 51 



The size and prominence of this keel are excellent 

 indications as to the power of prolonged and steady 

 flight. 



Many other examples might be given in illustration 

 of the principle that, among the Vertebrates, we are 

 dealing with countless modifications of a single type, 

 with lower and higher grades of organization and 

 adaptation to a great variety of habits of life, ter- 

 restrial, burrowing, climbing, swimming, flying, etc., 

 etc. The hypothesis of evolution offers the simplest 

 and most natural explanation of the facts. 



Similar considerations apply to the other great 

 types of animal structure. Among the Crustacea, 

 for example, we may take the common lobster, 

 which is one of the most advanced of these animals. 

 There is a large anterior shield, or carapace, which 

 is a single piece and, behind this is a number of 

 movable rings or segments, which compose the 

 abdomen or tail. To each segment is attached a 

 pair of jointed appendages, which are the swimmerets, 

 each of which is divided into external and internal 

 portions. The last pair of the swimmerets is very 

 much expanded and forms with the terminal seg- 

 ment a broad tail-fin. When the lower surface of 

 the lobster is examined, it is seen that the region 

 covered by the carapace is likewise composed of 

 segments, each with its pair of jointed appendages 

 but in this region the segments are immovably fixed 

 together and the appendages are altogether different 

 in appearance from those of the tail. The four 



