...frequently recurring struggle for existence, it follows that any being, if it vary in any manner Hfc profitable to itself, under the complex and sometimes...varying conditions of life, will have a better chance ofsun'ival and thus be naturally selected. From the strong principle of inheritance, any selected variety...

...possibly survive ; and as, consequently, there is a frequently recurring struggle (against disease) it follows that any being if it vary however slightly in any manner (unprofitable) to itself (and aiding the growth of parasites and bacteria) will have a better chance...