26 THE AGE OF THE EARTH 



quite the same as those of the Pharaohs. But putting 

 this on one side, the admitted constancy in some few 

 common forms proves very little, for so long as the 

 environment remains the same natural selection will 

 conserve the type, and, so far as we are able to judge, 

 conditions in Egypt have remained remarkably constant 

 for a long period. 



Change the conditions, and the resulting modification of 

 the species becomes manifest enough ; and in this connec- 

 tion it is only necessary to recall the remarkable mutations 

 observed and recorded by Professor Weldon in the case 

 of the crabs in Plymouth Harbour. In response to in- 

 creasing turbidity of the sea water these crabs have 

 undergone or are undergoing a change in the relative 

 dimensions of the carapace, which is persistent, in one 

 direction, and rapid enough to be determined by measure- 

 ments made at intervals of a few years. 



Again, animals do not all change their characters 

 at the same rate : some are stable in spite of changing 

 conditions, and these have been cited to prove that none 

 of the periods we look upon as probable, not twenty-five, 

 not a hundred millions of years, scarce any period short 

 of eternity, is sufficient to account for the evolution of 

 the living world. If the little tongue-shell, Lingula,* 

 has endured with next to no perceptible change from the 

 Cambrian down to the present day, how long, it is some- 

 times inquired, would it require for the evolution of 

 the rest of the animal kingdom ? The reply is simple : 

 the cases are dissimilar, and the same record which 

 assures us of the persistency of the Lingula tells us in 

 language equally emphatic of the course of evolution 



* In connection with the longevity of this genus, may be men- 

 tioned the extraordinary tenacity of life exhibited by the individual ; 

 its resistance to unfavourable conditions has been remarked on by 

 Morse and other observers. 



