276 CAUSES OF VARIATION 



the supposition being that sugar is the special .attracting sub- 

 stance in the latter case. It is noteworthy that the pollen tube 

 is attracted not simply to its own stigma but also to the pistil 

 and the ovule of other species, even of a different genus with 

 which it is unable to unite. 1 Davenport says : 2 



The results of experimentation upon chemotropism show that various 

 substances may direct the growth of such elongated organs as tendrils, roots, 

 and hyphae of plants. ... In many instances it can be shown that the 

 direction of growth is on the whole advantageous to the organism. ... In 

 other cases, however, the response seems to have no relation to adaptation. 



The immediate cause of change of direction is " excessive 

 growth on one side, due to excessive imbibition or to excessive 

 or restricted assimilative activity." 



The evidence seems conclusive that the chemical elements 

 constituting living matter have not entirely lost their ordinary 

 affinities and properties ; that protoplasm is in many respects 

 subject to the laws of other chemical compounds ; that its activ- 

 ities may be accelerated or retarded ; that they may be tempo- 

 rarily or even permanently modified in. character by chemical 

 alteration, or even entirely destroyed by entering into new and 

 strange combinations with surrounding substances. Here is a 

 real cause, at least of occasional variation, possibly of those 

 sudden changes we call mutation. 



Rhythmical contraction of muscle instituted by certain chem- 

 ical substances. The irritability and consequent contraction of 

 muscles due to influences of a chemical nature have already been 

 noticed, as in the case of the movement of the tentacles of the 

 actinian, the attractive or repulsive effect of certain odors, and, 

 to some extent, in the placing of insect eggs in " exactly the 

 right spot." 



It is now well known that certain salts exert a specific action 

 upon muscle, exciting even rhythmic contraction. As long ago 

 as 1 88 1 Biedermann discovered that if the muscle of a frog be 

 carefully excised at a low temperature (o-io C.), then weighted 

 and dipped into a 0.6 per cent solution of sodium chlorid con- 

 taining also small amounts of sodium phosphate and sodium 



1 C. B. Davenport, Experimental Morphology, p. 339. 2 Ibid. p. 343. 



