212 EVOLUTION, OLD AND NEW. 



cannot be performed simply from irritation, because 

 cold and darkness are negative quantities, and on that 

 account sensation, or volition are implied, and in con- 

 sequence a sensorium or union of their nerves. So 

 when we go into the light we contract the iris; not 

 from any stimulus of the light on the fine muscles of 

 the iris, but from its motions being associated with the 

 sensation of too much light upon the retina, which 

 could not take place without a sensorium or centre of 

 union of the nerves of the iris, with those of vision. * 



"Besides these organs of sense, which distinguish 

 cold, moisture, and darkness, the leaves of mimosa, and 

 of dionsea, and of drosera, and the stamens of many 

 flowers, as of the berbery, and the numerous class of 

 syngenesia, are sensible to mechanic impact, that is, 

 they possess a sense of touch, as well as a common 

 sensorium, by the medium of which their muscles are 

 excited into action. Lastly, in many flowers the an- 

 thers, when mature, approach the stigma, in others the 

 female organ approaches to the male. In a plant of col- 

 linsonia, a branch of which is now before me, the two 

 yellow stamens are about three-eighths of an inch high, 

 and diverge from each other at an angle of about fifteen 

 degrees, the purple style is half an inch high, and in 

 some flowers is now applied to the stamen on the right 

 hand, and in others to that of the left ; and will, I 

 suppose, change place to-morrow in those, where the 

 anthers have not yet effused their powder. 



"I ask by what means are the anthers in many 

 flowers and stigmas in other flowers directed to find 

 *See ' Botanic Garden,' part i. cant 3, 1. 440, 



