36 ORGANIC BEHAVIOUR 
take any direct share in the continuity of the race. Here- 
ditary transmission is therefore restricted to the germinal 
substance of these reproductive cells. Trace the ancestry of 
any cell in the adult body, say a nerve-cell, and you reach the 
fertilized ovum. ‘Trace back the ancestral line yet further, 
and you follow a long sequence of reproductive cells, or, at 
least, of cells which have undergone but little differentiation ; 
but never again will you find, in the course of a genealogy of 
bewildering length, a nerve-cell. Such a tissue-element is a 
descendant, but cannot become an ancestor; it dies without 
direct heirs. 
It is universally admitted that the bodily structures are 
subject to what is termed modification under the stress of 
environing circumstances. The muscles may acquire unusual 
strength by use and exercise; the nerve-centres may learn 
certain tricks of behaviour in the course of individual life ; and 
other structures may be similarly accommodated to the con- 
ditions which affect them. To such modifications of structure 
or function in the organs or parts the term acquired is 
primarily applied. The tissues have thus a certain amount 
of organic plasticity, through which they are adjusted to a 
range of circumstances varying in extent. They are able to 
acquire new modes of behaviour. But the cells of which they 
are composed are off the line of racial descent. They leave 
no direct heirs. When the body dies the modifications of 
behaviour acquired by its parts perish with it. Only if in 
some way they exercise what we may term a homeopathic 
influence on the germinal substance can the accommodation 
they have learnt be transmitted in inheritance. By a homeo- 
pathic influence is here meant one that is of such a nature as 
to communicate to the germinal substance, the seeds of similar 
changes of structure or function. And of the occurrence of 
any such homeceopathie influence there is no convincing 
evidence. 
Logically contrasted with the modifications of the tissues, 
dependent on organic plasticity, are the variations which arise 
from the nature and ccnstitution of the reproductive cells, 
