1884.] Growth, its Conditions and Variations, 1217 
dividuals, would be very unlikely to leave a geological record of 
their existence. This perhaps is one important cause of that 
marked absence of link-forms between related species which has 
given rise to so much controversy. 
The effect of this influence upon animals of different type, but 
subsisting on similar food, would probably be of a different charac- 
ter. While acting to crowd out older forms it might also insti- 
gate specific variation, and the evolution of new organs and 
habits. Thus progress in any one type might powerfully tend to 
cause progress in other types, and new species evolve simultane- 
ously in several unrelated types, through the action of a single 
initial force, 
As to the succession of huge Mammalia in the Tertiary age, 
little need be said beyond the considerations already taken. One 
important fact appears, that the greater land Mammalia were 
Herbivora, a fact in opposition to that which appears in the case 
of air and ocean animals, in which elements the Carnivora have 
always held the supremacy in size. The reason for this we have 
considered in the case of the birds. As for the ocean animals it 
naturally arises from the fact that in the ocean animal food is far 
_ More abundant and nutritious than vegetable. On the land the 
*pposite conditions rule. Vegetable food is more abundant, 
While animal food can only be obtained with greater exertion 
than is required in the water. Hence we find the land Herbivora 
‘teadily tending to exceed the Carnivora in size. This is not the 
ase with those timid Herbivora which seek safety in flight, and 
thus exhaust tissue by great muscular effort. But as soon as a 
-grass or leaf-eating animal grew strong and bold enough to resist 
and fight off carnivorous foes, the diminished exertion required 
: and the greater time for nutrition, enabled it to increase ın roe and 
à quickly grow too powerful to dread the strongest CANE 
this result we find occasionally even among the ordinarily 
_ “mid deer, as in their extinct relative, the gigantic ——, 
i Of the other huge forms it will suffice to mention the mig ve 
Stalking Dinosauria of the reptilian age, whose biped vega 
. ‘Must have aided them to a superabundant supply of ste ad 
and the elephantoid mammalian type whose superior organization 
: “nabled them to persistently survive and to evolve 
_ .“t adapted forms, while other types swelled into hugeness 
_ “RK again and disappeared beside them. 
successively 
