IV 

 THEORIES OF HEREDITY 



The Presidential Address read at the Annual Meeting of the Midland 

 Union of Natural History Societies, held at Oxford, September 23, 1889. 

 Reprinted from the Midland Naturalist, November, 1889. 



Revised and greatly modified: in large part rewritten. 



IN order to understand the problem of heredity, it is 

 necessary to have some general idea of the manner in 

 which the higher organisms are built up. The lowest 

 organisms, both animal and vegetable (Protozoa and 

 Protophyta), consist of single cells, while all higher 

 animals and plants (Metazoa and Metaphyta) are com- 

 posed of cell aggregates. A single Protozoon does not 

 represent a single Metazoon, but one of the innumerable 

 units of which all except the minutest Metazoa are com- 

 posed. 



The higher animals are, however, something more 

 than aggregates of cells ; they are cell-republics, in which, 

 at any rate in health, the structure and function of the 

 units are subordinated to the good of the whole. Certain 

 diseases are due to the literal insubordination of some 

 of these units, which grow and multiply in defiance of 

 that relationship in proportion, in position, and in the 

 consumption of nutriment, which is necessary for the 

 well-being of the whole. The surest hope of successful 

 treatment lies in an early extirpation of the centre of 

 insurrection. Later on, the centre will not only grow, 

 but will dispatch agents along the channels of communi- 

 cation, setting up other centres of mischief in distant 

 parts of the body. Such a republic is not only liable to 

 destruction from within by the revolt of its own members, 

 but also by the successful attack of enemies from without. 

 Numerous other forms of life are ever seeking to obtain a 



