S64: PROVISIONAL HYPOTHESIS Chap. XXVII. 



middle of their development are widely different from one 

 another. Still more striking cases could be given with 

 respect to the Echinodermata. With the Medusas or jelly- 

 fishes Professor Allnian observes, " The classification of the 

 •• Hydroida would be a comparatively simple task if, as has 

 " been erroneously asserted, generically-identical medusoids 

 " always arose from generically-identical polypoids ; and, on 

 •• the other hand, that generically-identical polypoids always 

 " gave origin to generically-identical medusoids." So again, 

 Dr. Strethill Wright remarks, " In the life-history of the 

 '•Hydroiche any phase, planuloid, polypoid, or medusoid, may 

 " be absent." 32 



According to the belief now generally accepted by our best 

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

 instance, the Medusae or the Macrourous crustaceans, are 

 descended from a common progenitor. During their descent 

 they have diverged much in structure, but have retained 

 much in common ; and this has occurred, though they have 

 passed through and still pass through marvellously different 

 metamorphoses. This fact well illustrates how independent 

 each structure is from that which precedes and that which 

 follows it in the course of development. 



The Functional Independence of the Elements or Units of the 

 Body. — Physiologists agree that the whole organism consists 

 of a multitude of elemental parts, which are to a great extent 

 independent of one another. Each organ, says Claude 

 Bernard, 33 has its proper life, its autonomy ; it can develop 

 and reproduce itself independently of the adjoining tissues. 

 A great German authority, Yirchow, 34 asserts still more 

 emphatically that each system consists of an " enormous 



'* mass of minute centres of action Every element has 



" its own special action, and even though it derive its stimulus 

 " to activity from other parts, yet alone effects the actual 

 v - performance of duties Every single epithelial and 



32 Prof. Allman, in ' Annals and bv Sars. 

 Mag. of Nat. Hist.,' 3rd series, vol. " 33 ' Tissus Vivants,' 1866, p. 22. 



xiii., 1864-, p. 343; Dr. S. Wright, 34 'Cellular Pathology,' translat. 



ibid., vol. viii., 1861, p. 127. See by Dr. Chance, 1860, pp. 14, 18, 83, 



also p. 358 for analogous statements 460. 



