1884. ] Growth, its Conditions and Variations. 1213 
dominant forms, and grew in many instances to enormous bulk, 
far exceeding anything previously known among the inhabitants 
of the land. Step by step the animal world was learning the art 
of food taking. The lesson was slowly learned. The two ener- 
gies of escape and pursuit kept pace in development, each forcing 
the other upwards. The continued effort at escape or defence 
must have yielded steadily greater efficiency and variety. This 
in its turn rendered necessary the development of new and more 
efficient weapons and methods of assault. Thus has life been 
pushed ever upward, the Carnivora forcing their living food to 
superior development, and the food animals exerting a like influ- 
ence upon their carnivorous foes throughout the whole long reign 
of earthly life. 
But as in all preceding eras of life, during this age of reptiles 
their destined successors to dominion were slowly developing, in 
lowly forms, far below the huge reptilian monarchs. True birds 
gradually developed, and the flying reptiles disappeared before 
them. The feather proved superior to the membrane as a flying 
organ, and in the competition for food which succeeded, the 
membrane-winged creatures vanished. At a later period other 
membrane flyers, of mammalian organization, came into compe- 
tition with the feathered tribe. But they have failed to dispossess 
them. The bats have been restricted to a nocturnal life, and the 
birds still hold the diurnal empire of the air. 
- Such was not the case with the mammalian occupants of the 
other two fields of life, the land and the sea. Before their onset 
the wave of reptilian life rapidly sunk, and that of mammalian 
life as rapidly rose. This advance of the Mammalia to suprem- 
acy seems to have been a slow one, and was probably hotly con- 
tested by the strongly-armed, swift-moving and huge-sized rep- 
tiles. But as in all cases, a superior organization eventually sep 
the battle. The earliest mammals were of the lowly-organized 
Marsupial type. They seem to have been incompetent to cope 
with the powerful and vigorous reptiles, and after their first 
‘pearance, vanish from sight throughout the long period of the 
Cretaceous era, They probably continued too eget 
ave any strongly declared marks of their existence 1 e 
s. 
Just how or when the placental mammals appeared, ge 
. But their unquestionable superiority to the i 
