XV 



DEVELOPMENT 



319 



to travel in their individual development along the rails 

 laid down in the progress of the race. Thus Pencuus, 

 starting of course as an ovum at the unicellular level, 

 has to stop as it were at the first distinctively crustacean 

 station the Nauplius stage. After some change and 

 delay, it continues to progress, but again there is a halt 

 and a change at the 

 Zosea station. Finallv 



*/ 



there is another delay 

 at the Mysis stage 

 before the P emeus 

 reaches its destina- 

 tion. The crab, on 

 the other hand, stops 

 first at the Zosea sta- 

 tion, the lobster at 

 the Mysis station, 

 while the crayfish 

 though progressing 

 very gradually like 

 all the others, has if 

 the simile be not too 

 grotesque a through- 

 carriage all the way. 



One must be careful 

 not to press the idea 

 of recapitulation too 

 far, (1) because the 

 individual life-history 

 tends to skip stages 

 which occurred in the 

 ancestral progress ; 

 (2) because the young 

 animal may acquire 

 new characters which are peculiar to its own near lineage 

 and have little or no importance in connection with the 

 general evolution of its race ; (3) because, in short, the 

 resemblance between the individual and racial history 

 (so far as we know them) is general, not precise. Thus 

 we regard Nauplius and Zosea rather as adaptive larval 



FIG. 106. LIFE-HISTORY OF Pcnccus ; 

 THE ZO.EA. 



