Septembbe 18, 1908] 



SCIENCE 



377 



pendulation. Of the truth of such an assump- 

 tion it is held that the facts of paleontology, 

 as well as our knowledge of the distribution 

 and relationships of living forms, furnish an 

 encouraging amount of corroborative evi- 

 dence. 



With the two hypotheses of pendulation and 

 a concentration of a primitive fauna in the 

 tropics established or granted, we are all but 

 ready to follow the ensuing movements that 

 are conceived to have determined the manner 

 and direction of the dispersals of the past and 

 of to-day. A circumequatorial movement, 

 away from the vicinity of the swing poles 

 themselves and toward the Schwingungskreis, 

 is all that remains necessary in order to get 

 these populations of old within the " sphere of 

 influence " of the swing. This lateral move- 

 ment is conceived as practically a logical 

 necessity, those forms of life inhabiting the 

 hot zone, and especially those portions of it 

 with most equable temperature, i. e., about the 

 swing-poles, being forced as they increased in 

 numhers to begin an effort to extend their 

 range in a like temperature and so long as 

 no obstacles of the medium or other hind 

 stood in the way. How inevitable are the con- 

 sequences of pendulation to a " caught " popu- 

 lation, in the view of the author, once the pre- 

 ceding suppositions have been allowed, is evi- 

 dent from the following quotation : " An or- 

 ganism that has arisen under the Schwing- 

 ungskreis [or has migrated there or near 

 there] will as a consequence of pendulation 

 be mechanically removed from its proper 

 climate unless it moves out to the right and 

 left on its own parallel. . . . Its distribution 

 thus becomes discontinuous. It will inhabit 

 two separated areas [transversally sym- 

 metrical points] which lie one to each side 

 of the Schwingungskreis," the eastward or 

 westward migration only coming to a stop 

 when first a point is reached " which most 

 nearly resembles climatically the original 

 habitat." Among the more salient examples 

 of transversally symmetrical points offered by 

 the author are: the alligator, the sole Asiatic 

 species of which lives in the Tang Tse Kiang, 

 while the nearest related form found else- 

 where inhabits the lower Mississippi; the 



shovel-nosed sturgeons, of Turkestan and the 

 Mississippi Valley; Psephurus and Polyodon; 

 the dipnoans, Lepidosiren and Ceratodus; 

 species of Cryptobranchus, Limulus, Pleuro- 

 tomaria; points of maximal abundance of 

 landsnails, etc. The map (No. 5) exhibiting 

 the geographical relations of the most striking 

 of these cases of " transversal symmetry " adds 

 much clearness and force to this portion of the 

 text. The predictableness of location of these 

 transversally symmetrical points in the light 

 of a " Pendulations-mechanik " leads the 

 author to observe of the first expedition sent 

 out to search for dipnoans in South America, 

 that the unsuccessful search in the Amazon 

 region was the result of a " false scent " 

 wholly unnecessary had the biologists of those 

 days possessed our present knowledge. With- 

 out pendulation " what a keen hjrpothesis 

 were necessary," he asks, in order to furnish 

 so clear and certain an elucidation of these 

 instances! That all species or faunas caught 

 by the swing should thus move out laterally 

 from the Schwingungskreis and their little 

 (or much) modified descendants so come to 

 occupy such symmetrically situated habitats 

 is too much to be expected and is not a mean- 

 ing intentionally conveyed by the author. 

 The length of time required for a full pendu- 

 lation, for example, the interval between the 

 Jurassic and the Tertiary, would seem, for 

 one thing, to leave ample time for such of the 

 forms affected as might choose to remain sta- 

 tionary to adapt themselves to the changed 

 conditions. And, again, as we shall see, the 

 ways in which pendulation may work in in- 

 fluencing the changes and movements of 

 organisms are more than one. 



That mode of distribution called by the 

 author " meridial symmetry " is another in- 

 teresting phenomenon on which light is 

 thrown by the pendulation theory. Thanks to 

 it, the occurrence of closely related forms, for 

 example, in California and Chili and in Japan 

 and Australia need no longer puzzle. " Forms 

 once carried northward with the polar swing, 

 in the next equatorial phase are carried into 

 too warm a temperature, and either move out 

 eastward or westward or ascend mountains. 

 On the latter they can go further south and 



