98 ESSAYS OF A BIOLOGIST 



impregnable sanction for his theories, is in reality 

 acting immorally because not attempting to envisage 

 the whole problem. 



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 hu- 

 man 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 ex- 

 tricate it from difllculties, it perishes. The first step 

 towards gaining is the power of learning. "Once bit- 

 ten, 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 substi- 

 tuted as subject of the proverb). 



When, as in the higher mammals, the power of 

 learning by experience is rapid, the individual or- 

 ganism is better able to adjust itself to the dangers 

 of life, and once more there is less sacrifice of indi- 

 viduals in the struggle. The same organism per- 

 sists: but of two possible types of behaviour, the un- 

 modified innate type is eliminated, the type modi- 

 fied 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 



