Variation and Natural Selection. 117 



if not more luxurious, mode of life. Then came hard times. 

 Others, in Africa and elsewhere, learnt the secret of ostrich- 

 farming. Competition brought down profits, and elimina- 

 tion set in — of which variety need hardly be stated. 



I believe that the alternation of good times and hard 

 times, during secular changes of climate and alternate ex- 

 pansions and contractions of life-areas through geological 

 upheavals and depression of the land, has been a factor of 

 the very greatest importance in the evolution of varied and 

 divergent forms of life, and in the elimination of inter- 

 mediate forms between adaptive variations. It now only 

 remains in this chapter to say a few words concerning con- 

 vergence, adaptation, and progress. 



Convergence, which is the converse of divergence, is 

 brought about through the adaptation of different forms 

 of life to similar conditions of existence. The somewhat 

 similar form of the body and fin-like limbs of fishes, of 

 ancient reptiles (the ichthyosaurus and its allies), of whales, 

 seals, and manatees, is a case in point. Both birds, bats, 

 and pterodactyls have keeled breastbones for the attach- 

 ment of the large muscles for flight. A whole series of 

 analogous adaptations, as the result of analogous modes of 

 life, are found in the placental mammals of Europe and 

 Asia, on the one hand, and the marsupial forms of Australia 

 on the other hand. The flying squirrel answers to the 

 flying phalanger, the fox to the vulpine phalanger, the bear 

 to the koala, the badger to the pouched badger, the rabbit 

 to the bandicoot, the wolverine to the Tasmanian devil, the 

 weasel to the pouched weasel, the rats and mice to the 

 kangaroo rats and mice, and so on. A familiar example 

 of convergence is to be seen in our swallows and martins, 

 on the one hand, and the swifts on the other. Notwith- 

 standing their superficial similarity in external form and 

 habits, they are now generally regarded as belonging to 

 distinct orders of birds. 



These are examples of convergence.* Animals of 



* Two more teshnical examples may be noticed in a note. (1) Professor 

 Ilaeckel has recently {Challenger Reports, vol. xxviii.) shown that the 



