



368 



PROVISIONAL HYPOTHESIS 



Chap. XXVII. 



Miiller remarks, the river cray-fish is hatched under the same 



: retains; the young lobster has 



form which it ever afterward 



divided legs, like a My 



the Palsemon 



under the 



: and 



form of a Zoea, and Peneus under the Nauplius-form ; 

 how wonderfully these larval forms differ from each othe 



known to every 



16 



Some other crustaceans, as the 



same author observes, start from the same point and arriv 

 nearly the same end, but in the middle of their development 



dely different from each oth 



Still more striking cases 



could be given with respect to the Echinodermata. With the 



Medusae or jelly-fishes Professor Allman observes, " the classi- 

 " fi cation of the Hydroida would be a comparatively simple 

 " task if, as has been erroneously asserted, generically-identical 

 ft medusoids always arose from generically-identical polypoids ; 

 " and on the other hand, that generically-identical polypoids 

 u always gave origin to generically-identical medusoids." So, 

 again, Dr. Strethill Wright remarks, " in the 



Hydroidae any ph 

 be absent." n 



planuloid, polypoid 



life-history of the 

 3r medusoid, may 



According to the belief now generally accepted by our best 

 naturalists, all the members of the same order or class, the 

 Macrourous crustaceans for instance, are descended from a 

 common progenitor. During their descent they have diverged 



much in st] 

 this diverg 



but have retained much in common 



and 



and retention of character has been effected 



though they have passed and 

 different metamorphoses. 



pass through 



This fact 



marvel lously 

 illustrates how ind 





pendent each structure must be from that which, precedes and 

 follows it in the course of development. 



The Functional Independence of the Elements or Ui 



yfth 



Body. — Physiologists agree that the whole organism consists of 

 a multitude of elemental parts, which are to a 

 independent of each other. Each organ, says Claude Bernard 



great extent 



16 Fritz Miiller's ' Fur Darwin,' 1864, 



Prof. Allman, in * Annals and Map:. 



s. 65, 71. The highest authority on cms- of Nat. Hist,,' 3rd series, vol. xiii., 1864, 

 taceans, Prof. Milne Edwards, insists 



( 



Sci. Nat.,' 2nd series 



Zoolog., torn. iii. p. 322) on their meta- 

 morphoses differing even in closely 

 allied genera. 



p. 348; Dr. S. Wright, idem, vol. viii., 

 1861, p. 127. See also p. 358 for 

 analogous statements by Sars. 

 18 ' Tissus Vivants,' 1866, p. 22. 



. 



