EXTERNAL INFLUENCES AS CAUSES OF VARIATION 



2 73 



be affected. In the same way nearly all kinds of motile organ- 

 isms are now known to be influenced by a variety of chemical 

 substances. 



Lubbock has shown that ants retreat from essence of clove, 

 lavender water, etc., 1 placed within one fourth inch of their 

 path, and Loeb found that the larvae of flies creep towards a 

 piece of flesh brought nearer than 1.5 cm. Not only flesh and 

 decaying meat, but meat juice in a glass, will allure, while fat 

 has no effect. Every farmer knows how quickly flies are 

 attracted by a dressed animal, and carrion birds by a carcass. 



According to Pfeffer's experiments the inorganic salts of 

 potassium, sodium, calcium, ammonium, magnesium, and many 

 other metals in 0.5 per cent solution act attractively upon Bac- 

 terium termo. " Inorganic acids ... in general act repulsively," 

 but phosphoric acid and the phosphates are strongly attractive. 

 Dewitz states that mammalian spermatozoa are attracted by 

 KOH. 



" Alcohol in grades between 10 per cent and i per cent acts 

 repulsively towards bacteria," but "glycerin is neutral." Malic 

 acid, which is of wide distribution among plants, is strongly 

 attractive to spermatozoids, even in a o.ooi per cent solution, - 

 a fact which is highly significant. 



This principle of chemotropism acting on higher organisms 

 gives rise to characteristic movements. In Loeb's experiments 

 on actinians 2 a piece of meat laid upon the tentacles so affected 

 them as to cause a bending which carried the meat into the 

 mouth, while a wad of water-soaked paper had no effect, but lay 

 there until removed. If, however, the paper was soaked in meat 

 juice it was received the same as a piece of real meat. (See 

 Fig. 29.) 



Now the actinian, consisting simply of a sac with a row of 

 tentacles around the edge, without brain or nerve centers of 



1 Experimenting upon means of preventing the ravages of the corn-root aphis, 

 Forbes of Illinois found that a small amount of oil of lemon on the seed corn, 

 before planting (costing but ten cents per acre), is able by its strong odor to 

 repel ants from the neighborhood of the corn hill for no less than six weeks 

 after planting. As the young aphis is absolutely dependent upon the attentions 

 of the ant, this treatment is effective. 



2 Loeb, Physiology of the Brain, pp. 49-50: 



