2 HEREDITARY CHARACTERS 



are an example of lifeless substance within the body. Such 

 dead material, however, has been produced by cells which 

 form part of the animal or plant during its life. 



Living matter, then, as far as we know, exists only in 

 the form of cells. There is no evidence at present available 

 to show that cells are, or can be, derived from anything but 

 pre-existing cells. Many attempts have been made to show 

 that living matter exists in a more primitive form, but when 

 carefully investigated they have all broken down. Theories, 

 of course, exist, but they are quite unsupported by any direct 

 evidence, and even that which is indirect suggests no more 

 than the possibility that living matter may have at some 

 time existed in a more simple form than it does at present. 



We may, for the sake of clearness, divide the whole of 

 the living organisms into two great groups : the unicellular 

 those in which an individual contains but a single cell ; and 

 the multicellular those in which the individual contains 

 more than one cell. The vast majority of the unicellular 

 organisms are invisible to the naked eye, but nevertheless 

 they perform the same functions of digestion, secretion, ex- 

 cretion, and so on, as the whole body of the multicellular 

 organism, which may be built up of many millions of cells. 



In some unicellular forms of whose life-cycles we have 

 an intimate knowledge, many generations of new individuals 

 are produced from pre-existing individuals by a simple pro- 

 cess of division. Thus in Stylonychia, one of the Infusoria, 

 simple division goes on for over a hundred generations. 

 Sooner or later, however, the individuals in the swarm begin 

 to conjugate. Two individuals adhere to each other, and 

 exchange portions of their nuclei in a somewhat complicated 

 manner. The individuals are all similar, there is no sex. 

 After conjugation has occurred, the animals separate, and 

 another period of simple division intervenes, during which 

 an enormous number of new individuals is produced from 

 the individual that conjugated. After a time, however, 

 some or all of a new generation of individuals again con- 

 jugate. We thus have a regular series of events. Com- 



