VIII] THE MAIDEN HAIR TREE 133 



whether in the far north or in the south, nor are we 

 able to explain the gradual decline of so venerable 

 and vigorous a race. 



As we search among the fragmentary herbaria 

 scattered through the sedimentar}^ rocks in that com- 

 paratively small portion of the earth's crust which is 

 accessible to investigation, we discover evidence of 

 a shifting of the balance of power among diiFerent 

 classes of plants in the course of our survey of suc- 

 cessive floras. Plants now insignificant and few in 

 number are found to be descendants of a long line of 

 ancestors stretching back to a remote antiquity when 

 they formed the dominant class. Others which flour- 

 ished in a former period no longer survive, either 

 themselves or in direct descendants. ' The extinction 

 of species has been involved in the most gratuitous 

 mystery.' We can only speculate vaguely as to the 

 cause of success or failure. Certain types were better 

 armed for the struggle for life, and produced de- 

 scendants able to hold their own and to perpetuate 

 the race through the ages in an unbroken line. 

 Others had a shorter life and fell out of the ranks of 

 the advancing and ever changing army. To quote 

 Darwin's words : ' We need not marvel at extinction ; 

 if we must marvel, let it be at our own presumption 

 in imagining for a moment that we understand the 

 many complex contingencies on which the existence 

 of each species depends.' 



