AUTHOR’S PREFACE.
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 occasion some distinctions, which may be explained as the consequences 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 elementary 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
