Epilogue: What Comes of Studying Vertebrates 79o 



tainly not from teleost fishes, but probably from less specialized 

 crossopterygians (or possibly Dipnoi?). Early reptiles were long of 

 body and tail and short-legged, resembling urodele amphibians, not 

 frogs and toads. The theromorph reptiles, transitional to mammals, 

 existed before reptiles had become differentiated into their several 

 specialized orders. 



It seems evident that specialization is acquired at the expense of 

 that plasticity which enables animals to become adapted to new situa- 

 tions and new modes of living. There is perhaps some remote analogy 

 to old age in the individual animal. An old saying asserts that "it is 

 hard to teach an old dog new tricks." Old persons do not readily change 

 their habits or adopt new ideas. They become "set." It has been sug- 

 gested that a highly specialized group of animals has arrived at a 

 stage of "phylogenetic old age." Such an "old" group may go on 

 indefinitely so long as external circumstances do not alter, but climatic 

 or other environmental changes would cause the extinction of a group 

 so "set" that it could not readapt itself to the new conditions. It 

 seems likely that teleost fishes, anuran amphibians, snakes, birds, and 

 cetaceans have nearly or quite reached the limits of their several lines 

 of specialization. It is difficult to imagine any anatomic improvements 

 which would better adapt a brook trout, tarpon, or whale to its aquatic 

 life, or which would increase the efficiency of the wings of an eagle or 

 sea gull. 



Long survival of a group is not necessarily dependent on high 

 specialization. Many small, feeble, and relatively unspecialized verte- 

 brates survive by avoiding competition. The little amphibian newts 

 and numerous mammals such as small rodents and insectivores survive 

 by keeping out of the way of potential enemies, just as the very early 

 mammals probably did. In short, animals may succeed and survive 

 for a time by becoming large, strong, aggressive, and dominant, or 

 they may survive by retreat. Some animals are specialized for retreat. 

 In burrowing animals the locomotor appendages may be highly special- 

 ized for digging. The auditory pinnae of burrowing mammals are more 

 or less reduced. 



Numerous instances of convergent evolution may be found along 

 two or more lines which are genetically very far apart. In some vivi- 

 parous sharks the wall of the oviduct produces either vascular folds 

 or processes (villi) which become closely related to the highly vascular 

 abdominal wall (wall of yolk-sac) of the embryo. By this means, the 

 nutritive and respiratory needs of the embryo are in part provided for 

 by diffusion of substances from the maternal blood to that of the 

 embryo. This arrangement is essentially like that of the mammalian 

 placenta. Some viviparous lizards (Genus Seps) are similarly "pla- 



