THE REPTILES AND THE BIRDS II9 



however, of relatively short duration: the climate, taken 

 through the whole of the period, was one simply of chilly 

 aridity, both cold and dry. 



With this drop in temperature the swamp-dwelling 

 flora, so rich in the Pennsylvanian, contracted to isolated 

 oases, the insects became smaller and more varied 

 but less abundant, and the Amphibia, that had fed 

 upon these insects and luxuriated in the Pennsylvanian 

 swamps, faced not only a scarcity of food and an excess 

 of chilliness and aridity, but another crisis that struck 

 at the very roots of their existence— their mode of repro- 

 duction. The adult might fight for smrvival in dwindling 

 pools and along seasonal streams by effort and cunning 

 in catching spiders and dragonflies, but for the eggs and 

 helpless young to be exposed in a highly restricted habi- 

 tat to ravenous fishes searching out every edible morsel 

 meant destruction. A frog lays many hundreds of eggs 

 each season, most of which under favorable conditions 

 may hatch into tadpoles with fish-tail and fish-gills, but 

 these must live in the water for several weeks imtil they 

 metamorphose into terrestrial-living adults that can es- 

 cape their aquatic enemies, and the probabihties are 

 great that in that interval all but a few will be devoiu-ed. 

 The adage about big fishes eating httle fishes is true 

 enough, but the fish tribe survives because the female 

 lays thousands of eggs— in some cases up to a million per 

 season. Nowhere is nature as careless of the individual 

 in order to preserve the race. The Amphibia, however, 

 lay far fewer eggs and under the pressure of narrowly 

 confined breeding pools the reproductive waste must 

 have increased enormously in the Permian until selection 

 favored those forms whose eggs could be deposited in 

 isolated hiding places on the land. In the extreme, this 

 meant that the egg must carry with it its own water sup- 

 ply, and it appears to have been in the amphibian subor- 

 der of the Embolomeri, and by forms akin to Seymouria 

 (see Figure 8) that the reproductive pattern departed 

 from the aquatic mode to produce the reptiles. As 



