REPRODUCTION. 247 



adaptive changes. (Furtlier differences in the theories of Nageli 

 and Darwin will be given below.) 



Nageli does not deny that we do not know hoio the stimuli act 

 upon the idioplasm to produce the required adaptive changes. But 

 the desire to formulate a theory of natural descent induced him to 

 supply the necessary connecting links from fantasy. For example, 

 he tries to explain in what manner the mechanical tissue-system was. 

 in all probability formed. Let us follow his argument. He says 

 that " tensions of pressure and of pulling are strongest where the 

 mechanical cells actually occur." Further, " tensions must predom- 

 inate in the elongated cells of the fibro- vascular bundles, because the 

 short parenchyma cells with their large intercellular spaces cannot so 

 readily resist these tensions, and would become displaced." The 

 question now arises, Why are ring-vessels, bast, and various cell-wall 

 thickenings formed ? According to Habeklandt and AjNibkonn, 

 the mechanical tissue with the vascular bundles is not always formed 

 from a common cambium. The well-established teaching of 

 ScHWENDENER, that the mechanical system is independent of other 

 systems in its arrangement ;ind position, does not harmonize with 

 JsTageii's conception. It is also well known that the mechanical 

 elements of the grass-internodes develop at the periphery, although 

 they are protected against stimuli of tensions by the leaf-sheath. 

 Further, in organs subject to bending, tensions are greatest at the 

 periphery. According to Niigeli. water-plants are evolved from or con- 

 verted into land-plants. Typical water-plants have a central bundle 

 of elongated elements. Before they can be changed into land-})lants 

 it is necessary for them to develop peripheral elongated elements. 

 Is it probable that the mutual aaaptation and arrangement of 

 mechanical and assimilating systems is due to blind mechanical 

 forces? Can any physiologist understand the complication which 

 arises when there is established an harmonious, rational equilibrium 

 between the position of mechanical and assimilating tissues at the 

 points of maximum tension and of favorable illumination ? This 

 much is certain, that the mechanical-physiological theory of descent 

 can here be no longer applied. 



Although Niigeli has allowed himself to be blinded by his love 

 for the theory of the natural origin of plants, yet his acute critical 

 powers are manifested in his attack on Darwin's theory of natural 

 selection. Darwin's theory of selection and Niigeli's theory of descent, 

 ■which he himself has designated as the theory of direct cause, have 



