14 ANIMAL LIFE AND HUMAN PKOGEESS 



its native element, regulates, by means of a beautiful mechan- 

 ism, the stream of water which circulates through the elaborate 

 system of canals by which it is penetrated, bringing the 

 necessary supply of food and carrying away the waste pro- 

 ducts. Even the amoeba, the commonly accepted type of 

 primitive simplicity, has the power of selecting those objects 

 which are suitable for food and rejecting those which are not, 

 and if it cannot control its environment to any greater extent 

 it can at least remove itself from injurious influences and seek 

 out the most favourable conditions for its existence. 



But this is not all ; were it so it would be difficult indeed to 

 explain the fact that living organisms as a whole are constantly 

 undergoing a slow, progressive evolution. From generation to 

 generation the progress is shght and for the most part imper- 

 ceptible, but in the long-run it has led to the development of 

 the most highly organised animals, even of man himself, from 

 the simplest beginnings, far simpler even than the humble 

 amoeba. This progressive evolution must be attributed 

 largely to the fact that all living organisms, even the lowest, 

 have the remarkable power of learning by experience, of profit- 

 ing by their mistakes and successes and doing better next time. 



The development of a complex animal from the unicellular 

 egg to the perfect individual, with aU its marvellous mechanism 

 and potentialities, is only possible because it is based upon the 

 accimaulated experience of a long chain of ancestors. How 

 that experience is handed on from generation to generation is 

 a question into which we cannot enter in this place. It is the 

 great problem of heredity, the most debated problem in the 

 whole realm of biological science. 



We do not question the value of experience in our own hves. 

 We know well enough that by experience we learn to perform 

 the most complex actions with ever-increasing facihty and 

 precision. It is perhaps not generally realised, however, that 

 the living organism actually builds up its own body by its 

 own activities. The first action of the developing egg is to 

 divide itself into daughter-ceUs, as its ancestors learnt to do 

 when they were unicellular Protozoa, and these daughter-cells 

 gradually nraltiply and differentiate themselves and combine 



