CLASSIFICATION AND ADAPTATION 5 



ment, and the name Zoaea disappeared altogether 

 as that of an independent genus, persisting only as a 

 convenient term for an important larval stage in the 

 development of crabs. 



These two kinds of study give us a knowledge 

 of the animals now living. But we find it a universal 

 rule that the individual animal is transitory, that 

 the duration of life, though varying from a few weeks 

 to more than a century, is limited, and that new 

 individuals arise by reproduction, and we have no 

 evidence that the series of successive generations 

 has ever been interrupted ; that is to say, the series 

 in any given individual or species may come to an 

 end ; species may be exterminated, but we know of 

 no instance of individuals coming into existence 

 except by the process of reproduction or generation 

 from pre-existing individuals. Further, we know 

 from the evidence of fossil remains that the animals 

 existing in former periods were very different from 

 those existing now, and that many of the exist- 

 ing forms, such as man, mammals, birds, bony 

 fishes, can only be traced back in the succession 

 of stratified rocks to the later strata or to those 

 about the middle of the series, evidence of their 

 existence in the periods represented by the most 

 ancient strata being entirely absent. Existing types 

 then must have arisen by evolution, by changes 

 occurring in the succession of generations. 



These three facts — namely, the limited duration 

 of individual life, the uninterrupted succession of 

 generations, and the differences of the existing 

 animals and plants from those of former geological 

 periods whose remains are preserved in stratified 

 rocks — are sufficient by themselves to prove that 



