THE PRINCIPLES OF BIOLOGY. 
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considerations this should be possible, seeing that every 
plant of whatever kind consists, at some stage of its 
existence, of a single vegetable cell; hence all the changes 
which it subsequently undergoes are in the nature of modi¬ 
fications impressed upon the organism either by external 
or internal causes. Could we classify and analyse internal 
forces in the same sense that those operating from the 
exterior are subject to classification and analysis the matter 
might become easy. 
The forces bearing upon and 'producing morphological 
differentiation Mr. Spencer considers to be :— 
(1) .— Growth, by altering the relations of the organism 
with the factors of nutrition, and therefore affecting different 
parts in differing degrees. 
A remarkable yet simple illustration of this is to be 
derived from our modern knowledge of the method of develop¬ 
ment of starch-grains. Adhering to, or half immersed in, 
the side of the “ feeder ” from which they derive their 
nourishment, their form at first is globular, for nutrition is 
practically equal in all parts. As they enlarge, the parts 
remote from the feeder receive less food than the proximal 
portions, and in proportion to their remoteness. Hence the 
form becomes progressively more and more eccentric, the 
eccentricity increasing in geometrical proportion. 
(2) .—Mutual influences, e.g., the pressure of new units 
on old ones, etc. Thus, two typically globular cells, passing 
into permanent union, would do so by flat contact-faces. 
Break a filamentous alga, and the flat end-walls of the 
cylindrical cells at the rupture-point will bulge and become 
hemispherical by removal of pressure. Again, very actively 
growing cells will stretch less actively growing cells in union 
with them. Hence the production of spiral vessels with 
exceedingly open spirals in the immediate neighbourhood of 
the pith of an actively growing stem. Again, the new units 
may cut off light from those older, and, at least in such cells 
as are dependent on light, will bring about manifest changes. 
In like way would come about the evolution,of a mechanical 
tissue. Compress the vital operations of a vegetable cell into 
smaller compass, and you produce equivalent increase in 
mechanical development. Hence the need to support a 
weight in itself induces ability to support it. 
To these two forces ought probably to be added another, 
at present, however, indefinable, but to which we can give the 
name of inherited tendency. Neither growth nor mutual 
influences can account for the development of, for example, 
hairs upon an epidermis. 
