BOTANIC EVIDENCE. 317 



seemingly have evidence that the last series of climatic changes has not been 

 a mere increase in aridity, broken by a period of uniformity, but has been a 

 pulsation from dry to moist and back again to dry. 



'"The whole history of Lake Otero and of the period since its disappearance 

 is a record of great and continuous climatic changes, with major fluctuations 

 indicated by the variations of the great ancient lake and its deposits. On these 

 fluctuations are superposed many series of minor pulsations, the greater of 

 which can be read in the triple record of chai^ng topography in lakes, dunes, 

 and arroyos. In general, it can be said that the Otero Basin shows the kind 

 of climatic fluctuations which Huntington's work has shown to be typical, 

 namely, large, long-period pulsations, upon which are superposed series after 

 series of smaller pulsations of less and less amplitude and shorter and shorter 

 period' (Free, 1914)." 



BOTANIC EVIDENCE. 



Plant fossils. — The fossil remains of plant life furnish direct and usually con- 

 vincing evidence as to past climates. This is necessarily based upon the 

 climatic relations of the same or related plants at present. In consequence, 

 the evidence is sometimes ambiguous, because we are still in doubt as to the 

 ecological significance of certain forms, such as the needle-leaf and the "bog- 

 xerophyte." The vegetative structure of the fossil plant must furnish the clue 

 to its functional behavior and hence to the habitat. But the latter may be in 

 the edaphic or developmental condition rather than in the climatic or climax 

 condition. Hence, the evidence of plant structures may be misleading, imless 

 interpreted in terms of succession. In the past as at present, the structure of 

 the leaf is of the first importance, though the evidence of the stem in terms of 

 wood, secondary growth, and size, and in assumed leaf-characters, is perhaps 

 equally significant. The distribution of fossil species and their differentiation 

 into floras seem to constitute a conclusive record of the extent and shifting 

 of the major climates. The restriction and expansion of floras mark periods 

 of great change, and the evolution of new floras must have been a direct if 

 gradual outcome of the greatest climatic changes of the past. 



Successional evidences. — From the extent of climax communities and the 

 nature of the life-forms which compose them, the fossil evidence has to do 

 largely with the climax and hence with the corresponding climate. This is 

 especially true of strates, in which only the more durable plant parts are ordi- 

 narily preserved. Moreover, since only a fragment of the population is 

 fossilized, the vastly greater abundance of climax individuals will have a decisive 

 effect. In the stase, the whole population is preserved in sequence, but this 

 is an aid only in recent peat stases, where layers and species are still clearly 

 defined. In general, all horizons of fossil plants testify primarily to the extent 

 and nature of the climax and its cUmate. Each, however, contains more or 

 less eAddence of successional stages and hence of edaphic conditions. The first 

 task in analysis is to recognize this material and to arrange it in the proper 

 sequence. Stases are of great help in this connection whenever the layers ai'e 

 sufficiently well-preserved to indicate the course of development. 



The evolution of a new climax vegetation is the outstanding effect of climate 

 and has already been used to mark the four great vegetation eras. Within 

 these a secondary climatic cycle is indicated by the differentiation and shifting 

 -of vegetation zones, such as mark a clisere. This in turn may be characterized 



