562 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.47. 



to and means for distribution are identical in both cases. It has long 

 ago been demonstrated that some Entomostraca are cosmopohtan, 

 while many are more or less localized, and there is every reason to 

 believe that the same holds good for the Kotatoria. A majority of 

 the species are, upon the basis of our present knowledge, limited to 

 particular localities. That this localization may be in some cases 

 only apparent is obvious; there is no doubt that many animals now 

 considered rare will eventually be shown to have a wide distribution. 

 But it should be evident that, if we begin by assuming universal 

 distribution, it is unUkely that anytMng will be discovered to dis- 

 prove it, not only because it is not searched for, but also on account 

 of no attempt being made to correlate the acquired evidence. 



That many species of Rotatoria undoubtedly do enjoy a world-wide 

 distribution is certain. This is not surprising, considering the great 

 age of this group, which, even if not demonstrated by paleontological 

 evidence, may reasonably be inferred from the well known fact that 

 the origin of Arthropods dates back to the unrecorded interval 

 between the Archaean and Paleozoic eras; that all the major divi- 

 sions of the lower Invertebrates originated long before the appearance 

 of the Artliropods is now universally accepted, and on tliis basis it is 

 not difficult to account for the extensive distribution of many species 

 of Rotatoria. In this connection it may not be out of place to call 

 attention to the fact that the earhest known bird, ArcJiaeopteryx, is 

 of Jurassic age, so that if the Rotatoria had been dependent on dis- 

 tribution by birds, they would have become extinct ages before dis- 

 persal could have occurred. It is also quite possible that bird 

 migrations may be of comparatively recent origin. 



For the reasons given, it would seem that the universal distribution 

 theory is at least unsafe as a working hypothesis, if not actually harm- 

 ful. A careful study of the rotatorian fauna of any locahty whatever 

 is still worth while, until we have far more complete records than we 

 now possess. Systematic work, including faunal lists, may not be 

 the most satisfying to the investigator, but both are the foundation 

 upon which the whole structure must be erected. 



One of the consequent drawbacks of the universal distribution 

 theory is that many forms without doubt remain unrecorded, if show- 

 ing a certain resemblance to some known species. As partly responsi- 

 ble for this one may also consider the imprecations so often hurled at 

 the head of the unfortunate who happens to give a new name to an 

 old species, surely a minor offense, provided the description and figure 

 are both good. It is very much easier to relegate a name to synonymy 

 than to sift out a number of species travehng under the same name, 

 and if merely fisted without description and figure it is obviously 

 impossible to determine the identity of the animal which the observer 

 had before him. 



