108 MEMOIRS OF THE NATIONAL ACADEMY OP SCIENCES. 



The alternative tlien lies between two possibilities : 



1. According to some, the forms of fresh-water fishes common to the now widely isolated lands 

 may have been transported afar and distributed in some unnatural manner, («) The progenitors of 

 the existing species may have been, for example, entombed in masses of ice, and such may have 

 been carried into the ocean and wafted to distant regions, where they may have found congenial 

 waters, been liberated from their long imprisonment, mated, and propagated their race, {h) They 

 may have even survived a long sojourn in salt water into which they wandered, (c) They may 

 have originated from congeneric species formerly existing in the ocean, but now extinct therein 

 and restricted to fresh water. 



2. According to others, community in type must be the expression of community of origin, and 

 the presence of fishes of long-established fresh-water types must imply continuity or at least 

 contiguity of the lands in the midst of which they occur at some, one time or other. 



Some physical geographers and thalassographers have found difficulty in admitting the 

 existence of land where now deep oceans exist, and have consequently denied the probability of 

 land connection between the southern lands. The objections do not appear to me to be insuper- 

 able. It is well known that the highest mountain chains are of comparatively recent geological 

 age. It remains then to consider which is the more i^robable, (1) that the types now common to 

 distant regions were distributed in some unnatural manner by means referred to, or (2) that they 

 are the descendants of forms once wide ranging over lands now submerged. 



The distribution of differeut types of fresh water fishes is analogous to the geographical range 

 of ainiihibians, mollusks, crustaceans, and other animals, and recent paleontological finds indicate 

 that even the Thylacinids (or at least forms resembling theui) were formerly natives of southern 

 America. While it may be admitted that it is possible that one or two types may liave been dis- 

 tributed in some such unnatural manner as has been suggested, it is highly improbable that all the 

 forms common to the distant regions could have been so distributed. 



The time at which immigration took place into the regions now occupied by the Galaxiids and 

 Haplochitonids can not be ascertained in the present stage of ignorance of the paleontological his- 

 tory of those forms. We can only conjecture from a knowledge of the paleontological history of 

 others. 



The various classes of animals differ remarkably in persistence of type or rapidity of change. 



On the one hand, echinoderms, mollusks, and articulates are very jiersistent in essential struc- 

 tural features. Of mollusks, for example, Unionids, Lymnseids, Valvatids, Melaniids, and Vivipa- 

 rids were not only represented by undoul)te(l members of the same families in Mesozoic times, biit 

 even genera had come intij existence which are still extant and were manifest iu species little diver- 

 gent from some which live in streams which coui'se lands in latitudes and longitudes where they 

 once floui'ished. 



On the other hand, mammals have undergone such changes that scarcely a smg\efu7nily which 

 existed in Eocene times has survived to the present, or, in otlier words, few, if any,* existing fam- 

 ilies can be traced back to the Eocene into forms which are now recognized as members of the 

 same family. 



It thus appears that in the same land, in early Tertiary times, mammals entirely unlike those 

 now living, and belonging to different families, walked the earth through which coursed streams iu 

 which flourished mollusks quite similar to those now living in succeeding courses, and the same 

 mammals might have crushed under foot gastropods, also almost undistinguishable from some still 

 existing.! 



Fishes to a certain extent are intermediate between invertebrates and mammals in persistence 

 of type, and consequently as indicators of geological periods. Few teleostonious families can be 

 traced farther back than Cretaceous times, and very few, if any, genera. 



In the present stage of science, then, we may be permitted to postulate (fishes being congeneric 

 in New Zealand, Australia, and South America), that there existed some terrestrial passage way 

 between the several regions at a time as late as the close of the Mesozoic period. The evidence 



'After a careful survej of all the famDies of mammals, I can not recognize a single one wbich now flourishes 

 which had undoubted representatives in the Eocene period, or at least of which unequivocal remains have heen found. 

 * As is well known laud shells like those of the present epoch lived as early as the Carboniferous. 



