pointed out, will form the conclusion of our history of the
anatomy of plants. It was a remarkable coincidence that this
molecular theory of organic forms, which is not without results
for zootomy also, was brought to completion at about the same
time, namely, the year 1860, that Darwin first published his
theory of descent. At the first glance the two theories seem
to have no connection with one another, and so the coincidence
in time appears to be quite accidental. But if we go deeper
into the matter, we find a resemblance between them which is
of great historical importance; they both of them exchange the
purely formal consideration of organic bodies, which had prevailed up to that time, for a consideration of causes ; as Darwin's
doctrine endeavours to account for the specific forms of animals and plants from the principles of inheritance and variability under the disturbing or favouring influence of external circumstances, so the object of Nageli's theory is to refer the growth and inner structure of organised bodies to chemical and mechanical processes. The future will show, whether the views which we owe to Nägeli will not contribute to the laying a deeper foundation for the theory of descent, since it is not improbable that a more thorough understanding of the molecular structure of organisms may add light and certainty to the still obscure conceptions of inheritance and variation.
The first beginnings were, as is usual in similar cases, small and inappreciable, and no one could have foreseen from the first observations of the facts in question what the ultimate development would be. We have said above, that von Mohl observed as early as 1836 the striation of certain cell-walls, and that this led Meyen, on the ground of further but to some extent inaccurate observations, to conceive of vegetable cell-walls as composed of spirally twisted threads. It was also noticed that von Mohl next distinguished true striation from spiral thickenings (1837), the two having been confused together by Meyen, and advanced so far as to form some idea of the molecular structure of cell-walls, without arriving however at
