How has the law of negligence been distinguished between negligence and negligence?

Salmond, at 322-23, distinguishes negligence as follows: (iii) Negligence Other writers have regarded the rule in Rylands v. Fletcher as a branch of the law of negligence. But it is perfectly clear that a man may be liable under the rule even though neither he nor anyone else has been guilty of any negligence in allowing the escape. It is equally clear that he may be liable though he has done no unlawful act in introducing the dangerous thing onto his land. In practice the importance of the distinction can be seen from the fact that if the plaintiff states his claim on the principle of Ry lands v. Fletcher he need only plead the escape; whereas if he states his claim in negligence he must plead and prove negligence. Nevertheless, it may be possible to regard these cases as a special instance of negligence, where the law exacts a degree of diligence so high as to amount practically to a guarantee of safety. The principle behind all these cases is that if a man takes a risk, which he ought not to take without also taking upon his own shoulders the consequences of that risk he must pay for any damage that ensues.