Mary is not at all slow to understand things, and when this hard-bodied little beetle, with a body like half a red-and-black pill, walked off the broom on to her hand, she guessed that he might have something to do with the torn-up egg cushions. So it didn't take her long to find another little beast like him actually nosing about in an egg sac and voraciously snapping up all the unfortunate tiny, red, black-legged baby scale insects. He ate the eggs, too, and seemed to take some bites at the mother insect herself, and then Mary found more of the lady-bird beetles, and still more. They were on all the broom bushes where the white cushions were. And so one of the dreadful tragedies going on in my garden was soon quite plain to Mary, and she was very sorry for the helpless white insects. "Where did the red beetles come from?" she asked pretty soon. "From Australia," I answered. "Or rather their great-great-grandparents did. These particular beetles were probably born right here in the garden, because a colony of them live here. But they couldn't if there were not some cottony cushion scale insects here too. For this particular kind of lady-bird beetle can't live on any other food--at least they don't--except this particular kind of scale insect and its eggs, which is surely a curious thing, isn't it?" But Mary is so used to finding that the insects have extremely unusual and curious habits--that is, habits different from ours--that she doesn't get excited any more when I tell her about them. She does though when she finds them out for herself, which makes me wonder if I haven't wasted a good deal of time in my life giving lectures to students about things instead of always making them find out for themselves. And maybe I am wasting some more time now while I am writing! "How did they come from Australia?" asks Mary. For she knows that Australia is several thousand miles away across the ocean from California, and lady-bird beetles do not swim. At least not from Australia to America. So I have to give Mary another informing lecture, and this is it: