PRESIDENT’S ADDRESS. 25 
communicated to adjacent cells and spread from cell to cell throughout 
the mass. An advance was made when the more impressionable cells 
threw out branching feelers amongst the other cells of the organism. 
Such feelers would convey the effects of stimuli with greater rapidity 
and directness to distant parts. They may at first have been retractile, 
in this respect resembling the long pseudopodia of certain Rhizopoda. 
When they became fixed they would be potential nerve-fibres and would 
represent the beginning of a nervous system. Even yet (as Ross 
Harrison has shown), in the course of development of nerve-fibres, each 
fibre makes its appearance as an amceboid cell-process which is at first 
retractile, but gradually grows into the position it is eventually to occupy 
and in which it will become fixed. 
In the further course of evolution a certain number of these 
specialised cells of the external layer sank below the general surface, 
partly perhaps for protection, partly for better nutrition: they became 
nerve-cells. ‘They remained connected with the surface by a prolonga- 
tion which became an afferent or sensory nerve-fibre, and through its 
termination between the cells of the general surface continued to 
receive the effects of external impressions; on the other hand, they 
continued to transmit these impressions to other, more distant cells by 
their efferent prolongations. In the further course of evolution the 
nervous system thus laid down became differentiated into distinct 
afferent, efferent, and intermediary portions. Once established, such 
a nervous system, however simple, must dominate the organism, 
since it would furnish a mechanism whereby the individual cells would 
work together more effectually for the mutual benefit of the whole. 
It is the development of the nervous system, although not proceeding 
in all classes along exactly the same lines, which is the most prominent 
feature of the evolution of the Metazoa. By and through it all impres- 
sions reaching the organism from the outside are translated into contrac- 
tion or some other form of cell-activity. Its formation has been the 
means of causing the complete divergence of the world of animals from 
the world of plants, none of which possess any trace of a nervous 
system. Plants react, it is true, to external impressions, and these 
impressions produce profound changes and even comparatively rapid 
and energetic movements in parts distant from the point of application 
of the stimulus—as in the well-known instance of the sensitive plant. 
But the impressions are in all cases propagated directly from cell to 
cell—not through the agency of nerve-fibres; and in the absence of 
anything corresponding to a nervous system it is not possible to suppose 
that any plant can ever acquire the least glimmer of intelligence. In 
animals, on the other hand, from a slight original modification of certain 
cells has directly proceeded in the course of evolution the elaborate 
structure of the nervous system with all its varied and complex func- 
