"j^i Charles Darwin. 



evidence of rarity preceding extinction is more strik- 

 ing in the successive tertiary strata, as remarked by 

 several able observers ; it has often been found that 

 a shell very common in a tertiary stratum is now 

 most rare, and has even long been thought to be 

 extinct. If, then, as appears probable, species first 

 become rare and then extinct — if the too rapid 

 increase of every species, even the most favoured, is 

 steadily checked, as we must admit, though how and 

 when it is hard to say — and if we see, without the 

 smallest surprise, though unable to assign the pre- 

 cise reason, one species abundant and another closely 

 allied species rare in the same district, why should 

 we feel such great astonishment at the rarity being 

 carried a step further to extinction ? An action going 

 on on every side of us, and yet barely appreciable, 

 might surely be carried a little further without ex- 

 citing our observation. Who would feel any great 

 surprise at hearing that the Megalonyx was formerly 

 rare compared with the Megatherium, or that one of 

 the fossil monkeys was few in number compared 

 with one of the now living monkeys? and yet in 

 this comparative rarity we should have the plainest 

 evidence of less favourable conditions for their exist- 

 ence. To admit that species generally become rare 

 before they become extinct — to feel no surprise at 

 the comparative rarity of one species with another, 

 and yet to call in some extraordinary agent and to 

 marvel greatly when a species ceases to exist, ap- 

 pears to me much the same as to admit that sick- 

 ness in the individual is the prelude to death — to 

 feel no surprise at sickness — but when the sick man 



