The highest, the mammals, adhere to this diet for a considerable time; they live by the maternal milk, rich in casein, another isomer of albumen.

The gramnivorous nestling is fed first upon worms and grubs, which are best adapted to the delicacy of its stomach; many newly born creatures among the lower orders, being immediately left to their own devices, live on animal diet.

In this way the original method of alimentation is continued--the method which builds flesh out of flesh and makes blood out of blood with no chemical processes but those of simple reconstruction.

In maturity, when the stomach is more robust, a vegetable diet may be adopted, involving a more complex chemistry, although the food itself is more easily obtained.

To milk succeeds fodder; to the worm, seeds and grain; to the dead or paralysed insects of the natal burrow, the nectar of flowers.
Here is a partial explanation of the double system of the Hymenoptera with their carnivorous larvae--the system of dead or paralysed insects followed by honey.