286 Origin of Biomes and Succession 



of the present terrestrial condition must have held. Except for the 

 areas of the earth's land surface having abundant year-around rain 

 and warm climates, the entire surface was uninhabited. Since that 

 time a remarkably varied and diverse biota has evolved and grad- 

 ually inhabited the colder and drier habitats. 



Following the tropical rain forest, the first of these new biomes 

 may have been a coniferous forest which existed in Pennsylvanian 

 time or even earlier and which adapted to cooler or drier climates 

 than the rain forest proper. Evidence of this forest consists of the 

 highly fragmentary fossils thought to be coniferous and assigned to 

 Taxeopsis, Taxodiella, Sboromirskia, Paleotaxus and other genera 

 (Andrews, 1955) and the primitive Pityales (Axelrod, 1959). Cer- 

 tainly fossil evidence suggests that such a coniferous forest existed 

 by early Mesozoic when genera occurred which are more typical 

 of conifers today (Knowlton, 1919) and which were the hosts of 

 wood-boring beetles belonging to existing families or superfamilies 

 (Ross, 1956Z?). 



At a still later time the more xeric grassland and desert biomes 

 arose, many elements of the desert biota in particular evolving into 

 highly specialized and, to our eyes, wierd forms such as the Cac- 

 taceae of the New World and the cactus-like Euphorbiaceae of 

 the Old World. 



In each new colonization the first requisite was a genetic change 

 in some pre-existing species which endowed its possessor with the 

 ability and behavior to live "farther out" in areas too inimical 

 ecologically for its parents to colonize. Once a species was able 

 to live in the new environment it would come under extreme selec- 

 tion pressures which would favor the establishment of any mutant 

 types better adapted to the new situation. In some instances such 

 conditions have doubtless led to unusually rapid evolution, per- 

 haps leading to the cactus-like desert plants or the highly spe- 

 cialized girafi^es and mammoths. The theoretical mechanics of such 

 evolution were considered in Chapters 4 and 5. 



A well-documented analysis of the colonization of a new area 

 was made by Axelrod (1958), in which he described the evolution 

 of the semi-desert biomes of southwestern North America from 

 ancestral elements of the tropical rain forest. According to his 

 hypothesis, somewhat arid conditions occurred in the Southwest 

 as early as Middle or Late Cretaceous time. By late Cretaceous 

 or Paleocene time species in such genera as Quercus, Ficus, Sahal, 

 Platanus, and Xanthoxylum had colonized at least the periphery of 

 these areas. These species would have had a greater tolerance to 



