MATTHEW, CLIMATE AND EVOLUTION 299 



If now we compare the general relations of tropical fresh-water fishes 

 with those of the North, it will appear very clearly that the highest and 

 latest in appearance of the several groups are still limited to the northern 

 world, and that, in the tropics, more primitive groups exist, many of 

 them known to be former residents of the northern world, others much 

 nearer to known or inferred ancestral groups than are any members of 

 the present northern fish fauna. Where the environment favors, some of 

 these groups have branclied out into an immense variety and number, far 

 exceeding what is known in the colder north. But they are distinctly 

 less progressive. In the southern continents, we meet with some remark- 

 able parallelisms to the dominant types of the North, very suggestive at 

 first of Antarctic connections, but probably explainable (as in G ataxias) 

 in other ways. These groups impress one as highly progressive, although 

 less so than the northern groups; but they do not appear to have con- 

 tributed materially to the tropical faim®. 



In some respects the fresh-water fishes present nearer analogies to the 

 birds than to mammals in their distribution; and this is no doubt con- 

 ditioned by their less strict limitation to land connections for their mi- 

 gration, and to the greater antiquity of the class. 



General Considerations on the Distribution of Invertebrates 



AND Plants 



It would be unwise to attempt any survey of the paleogeographic data 

 afforded by invertebrates and plants. Lacking both tlie special knowl- 

 edge necessary for a critical consideration of the data, and the time neces- 

 sary to make even an adequate compilation, it would add nothing to the 

 argument. While, for reasons already given (page 272), placing most 

 weight on the evidence obtainable from mammals, I fully recognize the 

 importance and variety of evidence outside the Vertebrata, and the force 

 which attaches to cumulative evidence from several independent sources. 

 At the same time I must express a strong conviction that the sources are 

 not really independent, and that concordant results in several groups 

 which flatly contradict the results obtained by a study of mammals, can 

 only indicate one of two things. Either the interpretation of the evi- 

 dence among the Vertebrata is incorrect or there are factors of error 

 common to the interpretation of the several other groups which accord 

 in their disagreement. What these factors may be, I have already indi- 

 cated and have attempted to show that they account for discordant results 

 based upon the distribution of the lower vertebrates and interpreted as 

 involving radical changes between continental and abyssal regions which 



