THE PRIMARY 0AUSE8 OF OROANIG EVOLUTION. 321 



celled forms of life, wliich in the beginning led to the 

 origination of the different types of life or lines of develop- 

 ment, the more generalized and primitive forms giving rise 

 to the later, more specialized forms. The most striking 

 cases are the evident origination of land vertebrates, with 

 limbs and lungs, from fresh-water ancestral forms, their 

 ancestors having in turn descended from marine forms. 

 Also the flying reptiles (pterodactyls) were evidently modi- 

 fications of creeping forms; the birds, so wonderfully 

 adapted to an aerial life, with little doubt descended from 

 creeping or walking vertebrates by a line parallel to that 

 of the reptiles. As the result of adaptation to water we 

 have the swimming and diving birds; the running birds, 

 such as the ostrich and moa bird, were evidently late off- 

 shoots from birds which had well-developed wings. The 

 causes were a change of halhts and of medium due to over- 

 stocking, and the consequent struggle for food and exist- 

 ence, obliging certain birds to forsake their ordinary 

 haunts where they could pick up seeds or insects, and to 

 live on shore-worms and mollusks or to dive for submerged 

 fresh-water or marine shellfish and plants. 



Among the mammals as soon as the type became estab- 

 lished it began to be differentiated into a great variety of 

 forms adapted to run or leap, to burrow, to climb, to swim. 

 The great marine saurians of past geological times are now 

 supposed to have diverged from terrestrial reptiles, and 

 the porpoises and whales from land mammals. This 

 differentiation of form and function, brought about in 

 adaptation to new needs and necessities by change of the 

 medium or environment; the overcrowding and struggle 

 for existence, resulting in the increased use of certain 

 organs and the disuse of others; the enforced permanent 

 isolation or segregation on land or in sea of the forms thus 

 originated; the migration of sea animals up rivers and their 

 isolation in land-locked bodies of water which gradually 

 became converted into fresh-water lakes; the necessity in 

 creeping forms of climbing trees either to escape their 



