A MONOGRAPH OF THE EXISTING CRINOIDS 13 



REVERSIBILITY OF DEVELOPMENT 



Gislen maintained that pinnulate forms have developed phylogenetically from 

 nonpinnulate types with extensive arm ramification. The holotomic recent types 

 are descended from the metatomic. Pinnules are thus simplified armlets or ramiculi. 

 That the tendency toward arm ramification is in all cases present, though it may be 

 latent, is shown by the fact that pinnules in certain cases abandon their character of 

 small unramified arms and develop into complete arms with pinnules. Gislen said 

 the appearance of pinnules can most easily be explained by assuming the existence of 

 obstructive factors on the removal of which the pinnule again develops into a ramified 

 arm, the phylogenetic development having presumably involved a great number of 

 obstructive mutations. From this point of view the appearance of axillaries with 

 two pinnules and cases of abnormal pinnulation otherwise difficult to explain are 

 easily accounted for as examples of two sided and reversed obstruction respectively. 

 In Metacrinus also obstruction of a temporary nature may be said to occur, retarding 

 the development of the inner arms so that they remain in the form of pinnules for a 



fairly long time. 



Dollo in 1893 laid down the law of irreversibility of development, which was later 



formulated by Abel as follows: "An aborted organ never recovers its former strength, 



and an organ that has disappeared never reappears— at least not developed in the 



same way." Gislen said that, broadly speaking, this rule is certainly correct. But 



it is also clear that the facts recorded and the conclusions to which they lead suggest 



that exceptions may be found to Dollo's law. From the point of view of heredity it 



may be maintained that Dollo's law is valid if the tendency to an organ has really 



disappeared— in that case a return to the original type would be out of the question. 



When a return really appears this may be interpreted as the removal of a factor of 



obstruction. Gislen said that this idea might perhaps be considered from the paleon- 



tological standpoint as rather perilous, though in reality it is not. As a matter of 



fact°such reversibility presumably occurs very seldom in nature. The reason for 



this would seem to be that the type of organization reached is the one that is most 



suitable from the point of view of selection and structure, and most economical; 



and it has often been reached in a roundabout way by an infinity of adaptations. 



That in certain cases the conditions necessary for "Riickschlage" really occur 



is shown, however, by the specimen with hypertrophied pinnules. Hypothetically 



the conditions might be pictured thus. In any collection of some thousands of 



comatulids there are always a number with one or another of their pinnules replaced 



by a more or less ramified arm. Thus in 270 specimens of Antedon petasus that 



Gislen secured at a single haul of the dredge off the Kristineberg zoological station 



at Flatholmen, three showed abnormal arm ramification. Assuming that these 



variations were of any advantage from the point of view of selection, and provided 



that they were inheritable, the old character would appear normally in all individuals 



after some few generations. 



Gislen said that there seems actuaUy to be a good example of a reversion of this 

 kind in the phylogeny of the comatulids. At one time, toward the end of the Paleo- 

 zoic, the proximal pinnules were suppressed by the disk, which rose high up between 

 the arms. They reappeared when the disk again retreated. But, according to 

 Gislen, there is still a relic of this development in the gap that appears in the pinnula- 



