THE MAIN LINES OF ANIMAL EVOLUTION 



earth. On the other hand, the mammals have evolved very rapidly during 

 the Cenozoic Era. 



Trends in Size. A very common trend in evolution, sometimes called 

 Cope's Law, is one toward increasing size of individuals. The original 

 studies of the phenomenon were made upon vertebrates, but comparable 

 studies have shown the same tendency in many groups of invertebrates 

 and plants. A review of the paleontology of almost any group shows that 

 its largest representatives are not its earliest ones, though not necessarily 

 its latest ones either. Newell has pointed out that species now living are 

 the largest known representatives of the vertebrates, crustaceans, echino- 

 derms, pelecypods, gastropods, cephalopods, and annelids. Yet the tend- 

 ency toward size increase has been by no means universal. As already 

 mentioned, the rise of herbs and shrubs is a recent thing, and they have 

 been derived from trees and other large plants. Hooijer has pointed out 

 that progressive size decrease has been characteristic of many vertebrate 

 groups during the Quaternary period, which is now in progress. 



Complexity and Efficiency. It is obvious that the general progress of 

 evolution has involved the development of new organ systems and in- 

 creasing complexity. Yet development of increased efficiency often in- 

 volves reduction in number and complexity of parts. Vestigial organs in 

 general could perhaps be considered in this light, but it is equally true 

 of actively functional structures. Thus the teeth of fishes are very numer- 

 ous and usually indefinitely replaceable. They are less numerous in am- 

 phibians and reptiles, still less so in mammals, where they reach their 

 maximum degree of specialization and efficiency. Much the same thing 

 is true of the vertebrae, and of the bones of the skull. This tendency can 

 also be exemplified by plants, for example by the reduction of numbers 

 of stamens in specialized plants. 



Increased efficiency is also often obtained by fusion of originally sepa- 

 rate parts. Thus the sacrum of mammals is formed by the fusion of three 

 to five originally separate vertebrae, thus making a much stronger attach- 

 ment of the hind limb to the vertebral column than would otherwise be 

 possible. Another good example is afforded by the pectoralis muscles 

 which, in tetrapods, arise near the midline of the chest region and insert 

 upon the humerus. Muscle slips from many adjacent body segments join 

 to form these muscles. Among plants, the corollas of flowers such as the 

 cucurbits or petunia are formed by the fusion of originally separate petals. 



Dollo's Law. Many times during the long history of life, advanced or- 

 ganisms have returned to ancestral habitats and modes of life. This gives 

 selective value to adaptations similar to those of the ancestral species, and 

 raises the question whether evolution might be reversible. Study of such 

 cases shows that always a gross similarity between ancestral and de- 

 scended structures is achieved without any genuine reversal at all. Thus 

 many reptiles and mammals have reverted to an aquatic mode of life. 

 They have assumed a generally stream-lined, fish-like form, and the limbs 

 have become shortened, webbed, and fin-like. Yet the skeleton of such 

 flippers is always distinctly that of the class to which the animal belongs 

 rather than that of a fish fin. Similarly, many angiosperms have returned 



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