THE CRETACEOUS PERIOD. 363 



final dominants. The greater the number of dominants the greater the 

 differentiating action of competition and reaction, and the greater the number 

 of serai forest stages before the climax is reached. Furthermore, it has been 

 shown that there is an intrinsic relation between development and structure, 

 as a consequence of which vegetation always shows structural differences. 

 On the one hand, these are serai and transient, on the other, they are repre- 

 sented by the great zones and alternes which indicate climatic conditions 

 similar to the edaphic ones for each associes. In short, it is felt that the suc- 

 cessional differentiation of life-form stages is itself a proof of the climatic 

 differentiation of climaxes in the Cretaceous. 



In accordance with the above, while it is granted that Cretaceous vegetation 

 was probably a more luxuriant mixture than is found in the center of the 

 deciduous climax of the United States to-day, it is assumed that Picea, Abies, 

 Pinus, Thuja, and Tsuga on the one hand, and Acer, Fagus, Hicoria, Juglans, 

 Quercus, etc., on the other, did not grow in climax association with Cycas, 

 Araucaria, Podocarpus, Ficus, Persea, Cinnamomum, Eucalyptus, etc. Con- 

 flicting as the geologic and ecologic evidence may be at present, the student of 

 succession must assume that the latter is controlling whenever the two are 

 not in agreement. As a consequence, the course of succession during the 

 Cretaceous is sketched upon the assumption that forest climaxes extended 

 over nearly all of the North American continent, that there were three such 

 climaxes, and that scrub and grassland climaxes existed in the highest latitudes 

 and altitudes and perhaps in interior arid basins. 



Cretaceous seres and coseres. — ^There is convincing evidence that succes- 

 sion dxu-ing the Cretaceous exhibited priseres, both hydroseres and xeroseres, 

 subseres and coseres. There appears to have been no significant clisere, though 

 the Flysch conglomerate of the Alps contains a suggestion of glaciation. 

 The hydrosere must have been the outstanding sere of aU the epochs. In the 

 Colorado and Montana, haloseres must have been especially characteristic, 

 while the oxyseres of peat-swamps must have been present throughout the 

 period, as shown by the distribution and abundance of coal-beds. The 

 pioneer submerged associes consisted of Chara, Lemna, and Potamogeton at 

 least, and it is probable that Batrachium, Ruppia, and Zannichella were present 

 also. The consequent reaction by filling led to the floating stage, marked by 

 Nelumbo, Castalia, Brasenia, Potamogeton, and Trapa in particular, and then 

 to the reed and sedge associes, characterized by Phragmites, Typha, Eguisetum, 

 Carex, and Cyperadtes, probably by Sdrpv^ and Juncus, and perhaps also by 

 Alisma, Sagittaria, Sparganium, Dryopteris, and Onodea. Sedgeland was prob- 

 ably succeeded primarily by heath or scrub, consisting of Betula, Myrica, 

 Andromeda, Kalmia, Cornus, etc. In more arid regions it would have jdelded 

 to grasses, and these to Quercus, Corylus, Crataegus, Ceanothus, Rhus, Prunus, 

 etc. In still other cases, reed-swamp or sedgeland would be invaded by Salix 

 and Populus, followed by Fraxinus and perhaps Ulmus also. The heath-scrub 

 would be replaced by Picea and Thuja, and probably Larix as well, and these 

 might pass iato a Pinus-Sequma climax. The latter may likewise have been 

 a stage leading to an Araucaria-Podocarpus climax, or to a mixed climax. 

 The drier scrub of Qv^rcus, Ceanothus, Rhus, etc., may have terminated in a 

 Pinus-Pseudotsuga climax, or have passed on into a deciduous climax in 

 which broad-leaved evergreens played a part. 



