Nov.. 1889. 
THEORIES OF HEREDITY. 
245 
THEORIES OF HEREDITY.* 
BY E. B. POULTON, M.A., F.R.S., 
PKESIDENX OP THE MIDLAND UNION OF NATURAL HISTORY SOCIETIES. 
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 
(Protozoa and Protophyta), both animal and vegetable, 
consist of single cells, while all higher animals and plants 
(Metazoa and Metaphyta) are composed of cell aggregates. 
A single Protozoon does not represent a single Metazoon, but 
one of the generally innumerable units of which the latter is 
composed. 
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 propor¬ 
tion and in the consumption of nutriment, which is necessary 
for the well-being of the whole. The surest hope of success¬ 
ful treatment lies in an early extirpation of the centre of 
insurrection. Later on, the centre will not only grow, but 
will despatch agents along the channels of communication, 
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 bv the 
successful attack of enemies from without. Numerous other 
forms of life are ever seeking to obtain a lodgment within it, 
and, if successful, discomfort, disease, or death, is almost 
invariably caused. The larger enemies, or parasites, have 
been known for ages; while the smaller, but far more 
dangerous foes, the germs of disease, have only been appre¬ 
ciated in comparatively recent times. Now, however, they 
attract a very large amount of attention, and the germ 
theory of disease is probably the most fruitful advance ever 
made in the history of medical and surgical science. 
The cells, or units, which compose the body of one of the 
higher animals differ greatly m structure according to the 
* Read at the Annual Meeting of the Midland Union of Natural 
History Societies, held at Oxford, September 23rd, 1889. [The Report 
of this Meeting, together with the discussion on the President’s 
Address and the Annual Report of the Union, will appear in our next 
number.—E ds. M. N.] 
