There is one very interesting evolutionary point which well illustrates the difference between pure biology and pure sociology, and yet emphasizes the natural connection between the two. Once again it has a connection with the greater flexibility of human mind. As we have seen, in the lowest animals behaviour is for the most part unvarying, hereditarily determined: the organism is capable of a number of definite reactions, and if these do not suffice to extricate it from difficulties, it perishes. The first step towards gaining is the power of learning. “Once bitten, twice shy” is applicable to all higher vertebrates; and it is not only the burnt child who dreads the fire (although a study of moths and candles will convince us that “Lepidopteran” cannot be substituted as subject of the proverb). When, as in the higher mammals, the power of learning by experience is rapid, the individual organism is better able to adjust itself to the dangers of life, and once more there is less sacrifice of individuals in the struggle. The same organism persists: but of two possible types of behaviour, the unmodified innate type is eliminated, the type modified by experience survives. If we like to put it in a way which is perhaps not wholly justifiable, there comes into being, besides the struggle for existence between individuals, a struggle for existence between different possible modes of reaction of one and the same organism. With the advent of man upon the scene, still new possibilities arise. First of all, he is capable of ideas, which, biologically speaking, are to be regarded as potentialities of behaviour. There is no evidence at present that even the highest animals possess ideas or even images.[25] Secondly, these ideas are transmissible by speech and writing, and accordingly tradition has come into being, so that modification of behaviour by experience can be operative not only within the individual life, not only from one generation to the next immediately succeeding, as in many mammals, but for an indefinite period. The experience of Moses, Archimedes, or Charlemagne, of Jesus, Newton, or James Watt is modifying our behaviour to-day.