EVOLUTION THE KESULT OF CHEMICAL FORCES. 101 



trees like the palms, that sprout from the ground with full-sized 

 trunk and grow only upward, to the exogens like the pines, with 

 bark and external rings of growth that often increase their bodies 

 to an enormous size. This great change happened, to all appear- 

 ance, suddenly in the Upper Silurian age. LeConte, in describ- 

 ing it as one of the steps of "rapid evolution," adds:* "When 

 all the conditions are favorable for a great advance, the advance 

 takes place at once, that is, with great comparative rapidity." 

 Another instance is the transition from the boneless, soft-bodied, 

 external shelled, invertebrates, to the internal skeletoned verte- 

 brates, with an entirely different nervous system. This happened 

 in the next succeeding age, the Devonian ; and the same author- 

 ity says of it : * " the advance is immense. It is impossible to 

 account for this unless we admit that, when conditions are favor- 

 able and the time is ripe for a particular change, it takes place 

 with exceptional rapidity, perhaps in a few generations." Other 

 instances are, the change from water-breathing to air-breathing 

 organs, from the fish skeleton to the reptilian, from the oviparous 

 orders to the mammalian, from marsupials to placentals, and 

 so on. 



There are definite and invariable lines of advancement in 

 which both animals and plants develop, even in widely separated 

 and independent provinces. All over the world are found 

 similar and nearly identical forms of life, both living and 

 extinct. Considering that impassable seas cover nearly three- 

 quarters of the surface of the earth, it is simply impossible that 

 all lands should in all ages have been so nearly connected that 

 species could pass from one to another and intermingle. There 

 must of necessity have been many centers of evolution, either 

 partial or complete. Not to specify others, I will merely take 

 the instance of the two polar regions. It has recently been very 

 clearly brought out that life began in the arctic zone,f at least 

 as one of the centers of distribution. But as the climatal and 

 continental conditions of the northern and southern hemispheres 

 alternate in certain long secular periods, and as there is an equal 



* "Elements of Geology" by Joseph LeConte, pages 317 and 333. 

 \" Where did Life Begin," by G. Hilton Scribner, 1884. 



