ORIGIN OF SPECIES 239 



But according to my theory, the immediate and 

 direct means whereby the appearance of higher life- 

 forms was made possible were the times which over 

 wide areas meant changes of the earth's surface and 

 the interruption of the life-process on land, times that 

 were followed by conditions which were suitable to 

 the new origin of life. 



It is a matter of common observation that life 

 flourishes in every nook and cranny that will support 

 it. When then at any one of several periods in 

 geologic history, the earth was crowded with the 

 highest forms of life which up to that time had 

 appeared (and with lower forms), not much further 

 advance was possible until there was opportunity for 

 a new beginning. The conditions that necessarily 

 followed the great upheavals and changes of the 

 earth's surface provided this opportunity. Each 

 great advance to higher life-forms was made possible 

 by a new beginning. 



Necessarily each succeeding recurrence of wide- 

 spread conditions suitable for the origin of life did 

 not, could not, provide these conditions uniformly 

 throughout, but only uniformly in a general way; 

 and since every slightest difference in constituents 

 or conditions necessarily is reflected, or registered, in 

 corresponding variation, there must have arisen great 

 varieties of forms which, on culmination, exhibited 



