PRINCIPLES OF VERTEBRATE MORPHOLOGY 13 



groups which have become adapted to a peculiar and limited en- 

 vironment, and of permitting only a few plastic, generalized types, 

 capable of adjusting themselves to the changed world conditions, to 

 persist. These generalized forms have then become the ancestral 

 stock from which the specialized types of the new period have arisen. 



There has usually been a period of struggle on the part of the per- 

 sisting generalized species to gain a foothold; then they have mul- 

 tiplied rapidly, and have entered upon a period of adaptive radia- 

 tion, which has resulted in the development of terrestrial, arboreal, 

 aquatic, fossorial, and volant types. Whether an animal is a fish, 

 amphibian, reptile, bird, or mammal, it meets a given set of life con- 

 ditions in much the same manner. The fish-like form, for example, has 

 been adopted not only by fishes, but by amphibians (several of the 

 persistently aquatic urodeles), by reptiles (notably by the extinct 

 ichthyosaurs) , by birds (penguins and the extinct Odontolcse), and 

 by mammals (whales and dugongs). These all have certain features 

 in common (Fig. 4) that are the fundamental adaptations for active 

 life in the water: the spindle-shaped body, fin-like appendages, 

 smooth, water-shedding exterior, tail-fin or hind limbs modified to 

 act as a propeller, and usually dorsal fins (in fish, amphibia, ichthy- 

 osaurs, and some whales) . Such structures that serve a similar func- 

 tion in adaptation to a given environmental complex are, for the 

 most part, not homologous, but merely analogous. They are techni- 

 cally called "homoplastic," which implies that they have been 

 molded out of diverse materials into like forms. 



Every successful vertebrate class has had a period of youth, when 

 the members were all comparatively generalized; a period of maturity, 

 characterized by adaptive deployment into all of the available life 

 zones; and a period of old age or senescence, characterized by the 

 development of bizarre, overspecialized types, incapable of weathering 

 a world crisis. The reptiles, for example, arose in the Palaeozoic and 

 were at first generalized lizard-like forms. Before the close of the 

 Palaeozoic there had arisen many specialized and a few precociously 

 senescent types, most of which became extinct during the troublous 

 climatic disturbances that ushered in the Mesozoic. Only a few of the 

 more generalized reptilian stocks survived the crisis and adjusted 

 themselves to the new conditions of the Mesozoic, the Golden Age 

 of the reptiles. This age saw the great second adaptive radiation of 

 the reptiles and the production of many overspecialized or senescent 



