328 HEROES OF SCIENCE. 



he missed his gentle and sympathizing wife. On 

 his return home Lyell received one of the Royal 

 Society medals for his work on the " Principles of 

 Geology," and in 1838 became President of the 

 Geological Society. About this time his atten- 

 tion was strongly drawn to the relative numbers of 

 living species found in the strata which had been 

 formed during the last geological or tertiary 

 age. In working at Sicily he had found that in 

 the latest beds in which the shells were hardly 

 fossilised, all the species were still living. That is 

 to say, he collected shells which were of course 

 dead, but they were similar to others which were 

 alive on the floor of the sea close by. The indi- 

 vidual had died, but the kind or species was still 

 alive. He examined the latest strata in England, 

 the crag of Norfolk, Suffolk, and Essex, and found 

 that the proportion of recent shells — that is to say, 

 of dead individuals belonging to living species — is 

 great. Some of the shells belonged to kinds 

 which are not now living, and are extinct. He 

 wrote, " I think we may lay it down as a rule, that 

 if any given tertiary deposit in which we have 

 found a few species of shells only, of which one 

 half, or a third, or even less, are recent, and those 

 recent ones inhabit the seas immediately adjoining, 

 the formation will be pliocene." This word was 

 one of three invented by Dr. Whewell, of Cam- 

 bridge, at Lyell's suggestion to explain the gradual 

 development of the recent animals and plants 

 during the past history of the globe. The other 



