Nature had to modify the size of modern sharks to the size of their modern victims. Modification of size has been consummated in every species of animal that could be bettered by it. The exceptions are solely among mammals and snakes.
A shark's size seems to depend solely upon how much food it gets and how old it is. You cannot tell how many years it has lived simply by its size. But the larger it is, the older it seems to be, is a safe assumption.
The porpoise is the one animal for which the shark has a deadly fear. The shark is the most tenacious of life of all animals. The great whale shark was brought ashore off King's Key, near Miami. The killing was bungled, the shark apparently dying, after five days endurance, only because of straining. He was harpooned and shot repeatedly without paying the slightest attention, as if unknowing of his wounds, he swam around in circles, normally, impervious to ropes and all resistance until stranded. A hole cut in his brain, through five inches of grist, did not kill him. That none of his captors knew the knife thrust in the throat act, at practiced in the North, is astonishing.
The viviparous shark, though not a mammal, suckles its young. The scientist still got the evidence that one of the many species of rays, the sting ray, suckled its young before birth from spirits and the spiral valve of the intestine, dripping with liquid akin to milk. 