-1 6 LECTURE XXX T. 



the theory of descent, according to which, organic forms are by no means fixed 

 from the beginning and simply copies of purely ideal types (Platonic ideas) : on 

 the contrary, the theory of descent states that every organic form is the result 

 of a historical process, in which the co-operation of two factors — the transmission 

 of peculiarities already acquired, and frequent small deviations from these, or varia- 

 tion — appear as causes of organic form. Of course there lies in this view the 

 recognition of a creation in time of organic forms, but it leaves the forces therein con- 

 cerned still in the main undetermined. The theory of descent demands in the first 

 place only the recognition of the fact that in the course of time organic forms have 

 been produced by some concatenation of causes ; it leaves to us, however, the 

 responsibility of the answer to the question what forces have been effective in 

 determining the production of organic forms in any given case. Some, to be 

 sure, imagine that heredity and variation are such forces ; but they forget that these 

 terms are simply words for facts which are not understood, and that heredity and 

 variation are not forces in the sense of Science in general — i. e. they are not causes 

 of motion. 



I premise these general considerations however, only with a view of throwing 

 light upon the apparently insignificant and trivial matters with w'hich we are here to 

 be concerned, and to make their significance clear from a general point of view. 

 So much is it at any rate obvious, that the processes of configuration in the vegetable 

 kingdom which are connected with growth, result from the co-operation of two 

 factors, the one of which lies in the quality of the organic material itself w^hich 

 is capable of configuration, while the other factor exists in the continuous influence 

 of universally co-operating cosmic forces, or in occasional accidental stimuli. To 

 employ an illustrative example taken from inorganic nature, I may remind the 

 reader how it depends on the physical nature of water that it becomes solidified in 

 hexagonal columns at low temperatures. In this capability the original nature of 

 the water is expressed; but it then depends upon accidental external circumstances 

 whether these hexagonal columns arrange themselves into a solid lump of ice, 

 apparently without any internal structure, or whether we have innumerable minute 

 hexagonal ice-crystals joined together on a window pane in the most various forms, as 

 so called ice-flowers, the growth of which depends upon differences in the external 

 conditions. As in this case, the organisable substance of the plant also seems to be 

 subject to certain unalterable formative forces, which however, under the influence 

 of accidental external causes, may experience the most difl"erent combinations and 

 variations. 



Proceeding now to a somewhat detailed description of some of the most interesting 

 observations appertaining here, it will be advisable to divide them into two groups, of 

 which the first comprises those cases where it is a matter of the origin of growing- 

 points, or of the influence of external forces on the formation of organs on these, 

 whereas a second group presents phenomena which are called forth by the influence 

 of external forces on post-embryonic growth. 



I. External influences on embryonic configiiraiio7i. I may here in the first 

 instance describe a case where, by the influence of an external force, gravitation, 

 the place at which the growing-points of new shoots are formed, is determined. 



Thiadiantha dubia belongs to the Cucumber family, and produces on its very 



