300 ANNALS NEW YORK ACADEMY OF SCIENCES 



are highly improhahle, to say the least, from a geological point of view, 

 fiind which are not merely unnecessary hut apparently impossible when 

 we attempt to explain tiie distribution of the higher vertebrates in accord- 

 ance with them. 



It is true that the evidence against such changes in prL'-Tertiary times 

 is less weighty, and that it diminishes further in the older periods of 

 geologic time. And the antiquity of many groups of imortebrates, 

 especially of land invertebrates, makes it impossible to limit the hypo- 

 thetical land bridges which their distribution is supposed U> rcquii'r. to 

 the Tertiary or even the Mesozoic. The permanency of the ocean basins 

 in the older geologic e])oclis is beyond the limits of this discussion. 



So far as a superficial acquaintance shows, the general distributional 

 relations of most land invertebrata and of plants appear to me to accord 

 with those of the mammalia. Primitive and archaic^^" types abound 

 chiefly in the tropics. The most progressive and dominant types are 

 Holarctic. The southern continents show common groups suggestive of 

 an Antarctic radiation, but which may, like the marsupials or the chryso- 

 chloroid insectivores, be remnants of formerly cosmopolitan groups whose 

 resemblance is due rather to persistence or to parallel evolution iinder 

 similar climatic stimulus than to such close atfinity as would involve 

 Antarctic continental connections. 



Where, as in the earthworms, we have no knowledge at all of their past 

 distribution, it is impossible to test this interpretation of their present 

 distribution; nor in such a group does it seem possible to estimate how 

 much and in what manner slow progressive climatic change might affect 

 their structural evolution, although climatic conditions are evidently 

 important in controlling their range. 



The point that I desire to emphasize is that, if such an interpretation 

 as I have suggested be possible, it should be accepted in preference to 

 one which would involve such unexplainable difficulties in the distribu- 

 tion of the higher animals and such improbable physiographic changes. 

 No hypothesis can be finally accepted that does not conform to tlie facts 

 of distribution in all groups of animals and plants. It is not a matter 

 of preponderant evidence. Every anomaly must be explained, e\ery dis- 

 tributional fact must be interpreted in accord with the rest, before we 

 can consider theories of paleogeography as conclusively proven. It is 

 not sufficient that the evidence in one group or in ten groups has been 

 interpreted on concordant lines, so long as there remains an eleventh 

 group which cannot be so interpreted. But, pending a final agreement 



. IS" Ai'chaic is used in the sense of divergently specialized but Httle progressive. 



