xxxn. 13 PARALLEL EVOLUTION 779 



tongue, large salivary glands, and other features. The hands of the 

 moles show how a similar result may be arrived at by various slightly 

 differing means (Fig. 514). 



There can be no doubt that vertebrates adopting a given mode of 

 life tend to acquire a particular structure. There is also evidence of 

 what we might call the converse, namely that animals with a particular 

 form tend to develop in certain directions. This is one of the forms of 

 the situation described by some as 'pre-adaptation'. The elasmobranch 

 fishes maintain equilibrium in the horizontal plane by a heterocercal 

 tail, driving the head downwards, and horizontal pectoral fins and 

 flattened front end of the body, having the opposite effect (p. 140). 

 From this type of organization creatures of ray-like type have several 

 times developed, flattened dorso-ventrally and obtaining their pro- 

 pulsive thrust from the pectoral fins. Conversely among teleosts, 

 where the compression is in a transverse plane, the bottom-living 

 types are flattened laterally, as in the sole or plaice. Where the swim- 

 bladder has been lost, however, there may be dorso-ventral flattening. 

 The more closely we examine the evolution of populations the 

 more signs of these similar tendencies in evolution appear. Parallel 

 evolution is so common that it is almost a rule that detailed study 

 of any group produces a confused taxonomy. Investigators are un- 

 able to distinguish populations that are parallel new developments 

 from those truly descended from each other. Examples we have 

 noticed from study of modern populations are the various types of 

 tree-living and burrowing anurans (p. 366). The 'tree-frog' and 

 'burrowing' conditions have been evolved both from true frogs and 

 from toads, probably several times in each case. Again, the habit of 

 burrowing underground, with loss of the limbs, has appeared in a 

 number of squamate reptiles; the slow- worms, amphisbaenas, and 

 subterranean skinks are certainly distinct lines, possibly each contains 

 more than one, and the snakes are probably derived from another 

 group that went underground. 



13. Some tendencies in vertebrate evolution 



These are only a few scattered examples, but they already suggest 

 that the evolutionary changes of vertebrate populations follow certain 

 patterns. Although in the history of each type there is no doubt that 

 much is unique, yet most types tend to follow along certain lines, 

 according to the situations they have reached. It is not possible yet 

 for the systematic morphologist to make any complete classification 

 or analysis of these tendencies. He can only point to a few of them, 



