AUTHOR’S PREFACE. XVil 
particles and a cell differs only in degree from that which exists 
between two cells; so also the principle of development in the 
latter can only then be similar, when it repeats itself in the 
rest of the elementary particles. I therefore quickly asserted 
this position also, so soon as I was convinced of the accordance 
between the cells of cartilage and those of plants in this sense. 
It now became easy to accommodate the principle which I 
had laid down to the rest of the tissues, since the principle 
itself had already made me acquainted with the law of their 
development. Actual observation also completely confirmed 
the conclusion which had been drawn with respect to the rest 
of the tissues. It was not absolutely necessary that this 
principle should recur in the elementary particles of vascular 
tissues; for since no independent vitality of the elements, 
and therefore no diversity in the fundamental powers of 
growth, was assumed in their case, so, without prejudice to 
the principle, might they be subject to entirely different laws 
of development. But slight as was the probability at the 
commencement, that the principle could be carried out with 
respect to them, observation soon showed that vessels do not 
establish any essential difference in growth, but merely occa- 
sion some distinctions, which may be explained as the con- 
sequences of a more minute distribution of the nutrient fluid ; 
of the change of material facilitated both by that means and 
by the circulation; and of a greater capacity of imbibition 
in the animal substance. Thus was the proposition firmly 
established by observation, that there is one common principle 
of development for the elementary particles of all organised 
bodies. It had already indeed been long known that all 
tissues were formed from a granulous mass; but that these 
granules bore some direct relation to the subsequent ele- 
mentary particles, and what that relation might be was known 
in respect to but a few of the particles, and in them the mode 
of development appeared to differ so much, that unity neither 
