This seems to be one of those cases where the ground of liability is to be sought in policy coupled with tradition, rather than in any form of blameworthiness, or the existence of such a chance to avoid doing the harm as a man is usually allowed. But the fact that remote inadvertence has been suggested for an explanation illustrates what has been said about the difficulty of deciding whether a given rule is founded on special grounds, or has been worked out within the sphere of negligence, when once a special rule has been laid down. It is further to be noticed that there is no question of the defendant's knowledge of the nature of tigers, although without that knowledge he cannot be said to have intelligently chosen to subject the community to danger. Here again even in the domain of knowledge the law applies its principle of averages. The fact that tigers and bears are dangerous is so generally known, that a man who keeps them is presumed to know their peculiarities. In other words, he does actually know that he has an animal with certain teeth, claws, and so forth, and he must find out the [156] rest of what an average member of the community would know, at his peril. What is true as to damages in general done by ferocious wild beasts is true as to a particular class of damages done by domestic cattle, namely, trespasses upon another's land. This has been dealt with in former Lectures, and it is therefore needless to do more than to recall it here, and to call attention to the distinction based on experience and policy between damage which is and that which is not of a kind to be expected. Cattle generally stray and damage cultivated land when they get upon it. They only exceptionally hurt human beings. I need not recur to the possible historical connection of either of these last forms of liability with the noxoe deditio, because, whether that origin is made out or not, the policy of the rule has been accepted as sound, and carried further in England within the last few years by the doctrine that a man who brings upon his land and keeps there anything likely to do mischief if it escape, must keep it in at his peril. /1/ The strictness of this principle will vary in different jurisdictions, as the balance varies between the advantages to the public and the dangers to individuals from the conduct in question. Danger of harm to others is not the only thing to be considered, as has been said already. The law allows some harms to be intentionally inflicted, and a fortiori some risks to be intentionally run. In some Western States a man is not required to keep his cattle fenced in. Some courts have refused to follow Rylands v. Fletcher. /2/ On the other hand, the principle has been applied to artificial [157] reservoirs of water, to cesspools, to accumulations of snow and ice upon a building by reason of the form of its roof, and to party walls. /1/