THE AMERICAN ENTOMOLOGIST. 



65 



animals, but, by a system of artificial selec- 

 tion which he to a great extent uncon- 

 sciously practices, he effects the most extra- 

 ordinary modifications in the plants them- 

 selves, illustrations of which it would be 

 quite superfluous to adduce. 



When to these considerations we add 

 those arising out of the relations which 

 insects sustain to plants we are brought 

 face to face with the most practical 

 questions of horticulture and agriculture, 

 a thorough acquaintance with which is of 

 the utmost importance. To this same 

 group also belongs the extensive subject of 

 galls. This study is essentially compound 

 and belongs to botany as well as to ento- 

 mology. The gall presents a case in which, 

 in the normal condition, the plant is the 

 loser, yet there is reason to believe that 

 such is not universally the case. The 

 whole subject has been so exclusively given 

 over to the entomologist that it may be 

 safely predicted that whenever it shall be 

 phytologically studied much valuable 

 knowledge will be added to that already 

 acquired. Plants are not generally killed 

 by the bite of the insect. This would 

 defeat the end the insect itself has in view. 

 The normal channels of circulation and 

 growth in the plant are simply blocked up 

 and forced to re-form under new conditions 

 favorable to the insect. The case is anal- 

 ogous to those in which man turns the 

 channels of streams to his own advantage or 

 in any manner diverts the forces of Nature 

 in his own favor, which is the essence of 

 invention and of all material progress. In 

 neither case is Nature actually defeated, it 

 is only controlled. In the gall the insect 

 has learned to control the forces of growth 

 to its own advantage. 



Where the same insect operates in the 

 same manner upon the same plant for many 

 generations a certain adaptation must take 

 place. The abnormal mode of growth 

 must become to a certain extent normal. 

 We may even suppose cases in which the 

 presence of the gall has come to be a 

 benefit rather than an injury. I have my- 

 self observed many cases in which it ap- 

 peared in no way to arrest the normal 



function of the plant. Specimens of 

 Eupatoriicm album recently collected ex- 

 hibited a swelling in the peduncle at the 

 base of each head, in the center of which 

 was a hollow cavity enclosing the larva. 

 But the tissue all round this cavity seemed 

 perfectly healthy and the flowers were in 

 all states of advancement and apparently 

 wholly unaffected by the gall. A remark- 

 able gall which I found on the prevailing 

 juniper of the Wasatch range, Juniperus 

 Californica, var. Utahensis, deceived every- 

 body but professional naturalists by its 

 resemblance to fruit, and though abundant 

 in certain localities, did not seem in the 

 least to impair the growth of the trees. 

 Oak galls, as all know, are green ; they 

 contain chlorophyl and must perform the 

 regular functions of assimilation. The 

 insect producing a gall may be regarded 

 as a parasite on the particular plant chosen 

 whose growing tissue the larvae usually 

 devour at something near the rate at which 

 it forms. There are cases both where 

 plants are parasitic on plants (e. g. lichens 

 on fungi*), and where animals are parasitic 

 on animals,! in which the foster individ- 

 ual is benefited, and that such should 

 sometimes be the case with plants regu- 

 larly affected by the same gall is not an- 

 tecedently improbable. 



With these few remarks on the first gen- 

 eral division of the subject or that in which 

 the dependence of animal life upon plant 

 life is shown, we may now pass to the 

 second division which includes the evi- 

 dence of a reciprocal dependence of the 

 plant upon the animal. 



That quadrupeds and birds are perpetually 

 conferring great service upon many forms 

 of plant life there is now no doubt. This 

 is done chiefly in securing their proper 

 distribution over the earth. Were the 

 seeds of plants compelled to depend upon 

 gravitation and the winds alone for their 

 distribution the relative proportions of the 

 various species would be very different 

 from that which we actually find. In 

 many cases the seeds, berries, and nuts. 



* Sachs, Lehrbuch der Botanik, p. 330. 



t Van Beneden, Animal Parasites and Messmates. 



