chap, iv.] and of Growth by Intussusception. 351 



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 i860, 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 pre- 

 vailed 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 circum- 

 stances, 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 Nageli will not contribute to the laving a 

 deeper foundation for the theory of descent, since it is not im- 

 probable 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 <>t 

 the molecular structure of cell-walls, without arriving however at 



