CHARACTER EVOLUTION 243 



process in mammals, let us observe the operation of Dollo's 

 law of alternate adaptation (p. 202) in the evolution of the tree 

 kangaroo {Dendrolagus) , belonging to the marsupial or pouched 

 division of the Mammalia. This is a case where many of the 

 intermediate stages are known to survive in existing types. 

 These tree kangaroos theoretically have passed through four 

 phases, as follows: (i) An arboreo- terrestrial phase, including 

 primitive marsupials like the opossum, with no special adap- 



AERIAL 

 AER9 ARBORt 

 ARBOREAL 

 ARB0R5TERRt 

 TERRESTRIAL 



H SPE'^IAlHED 

 FEET OF CLIMBING TYPE, GREAT TOE OPPOSABLE 

 i FOURTH TOE ENLARGED 



PRIMITIVE MARSUPIALS 



WITH NO SPECIAL ADAPTATIONS 



FOR CLIMBING 



Fig. 116. Four Phases of Alternating Adaptation ix the Kangaroo Marsupials, 

 According to Dollo's Law. 



1. Primitive arboreo-terrestrial phase — tree and ground living forms. 



2. Primitive arboreal phalanger phase — tree-living forms. 



3. Kangaroos — terrestrial, saltatorial phase — ground-living, jumping forms. 



4. Tree kangaroos — secondaril\' arboreal, climbing i)hase. 



tations for climbing; (2) a true arboreal phase of primitive tree 

 phalangers with the feet specialized for climbing purposes 

 through the opposability of the great toe (hallux), the fourth 

 toe enlarged; (3) a cursorial terrestrial phase, t^qpified by the 

 kangaroos, with feet of the leaping type, the big toe (hallux) 

 reduced or absent, the fourth toe greatly enlarged; (4) a second 

 arboreal phase, typified by the tree kangaroos {Dendrolagus), 

 with limbs fundamentally of the cursorial terrestrial leaping 

 type but superficially readapted for climbing purposes. It 

 is clear that there can be no internal perfecting tendency 

 or predetermination of the heredity-chromatin to anticipate 

 such a tortuous course of evolution from terrestrial into arbo- 

 real life, from arboreal back to a highly specialized terrestrial 



