NATURE AS DRAMA AND ENGINERY. ^97 



eral resemblance between the moth and the wasp ; any moth in 

 which that resemblance was in any degree unusually marked had 

 therein an advantage, and tended to be in some measure left alone 

 by its enemies ; in thus escaping it could transmit its peculiari- 

 ties of form and hue to its progeny, and so on, until in the rapid 

 succession of insect generations, amid the equally rapid destruc- 

 tion of comparatively unprotected moths, the present striking 

 similarity was at last attained. The study of mimicry of this 

 type has from an unexpected quarter afforded singular confirma- 

 tion of the theory of natural selection ; in many cases the evidence 

 of transformation within comparatively recent time is distinct 

 in the bee hawk moth, for example, the wings as they emerge 

 from the chrysalis are thinly clothed with scales of ancestral 

 derivation which are shaken off in the insect's first flight, with 

 the result that the bumblebee is the better and more gainfully 

 resembled. 



As in the study of insects, so in that of plants observation in 

 the field at every stage of growth and development is needful to 

 supplement the disclosures of the microscope and the dissecting 

 needle. Many species, of which the milkweed blossom may stand 

 as a type, are absolutely dependent on insects for their fertiliza- 

 tion. How, therefore, can they be fully known in the laboratory 

 and the herbarium ? There is no more remarkable adaj)tation in 

 Nature than that by which an orchid and the insect which con- 

 tinues its race conform to the outlines of each other. And hun- 

 dreds of flowers less consi3icuous than this orchid present per- 

 fumes, colors, and mechanism for attracting, seizing, and even 

 imprisoning their insect visitors, which might well be the work 

 of deliberate contrivance instead of inevitable selection from vary- 

 ing scents, hues, and forms of those which prove slightly more 

 serviceable than others. That clover, peas, and other legumes re- 

 ceive their nitrogen from the air has long been suspected by agri- 

 cultural chemists. The details of the process disclose one of the 

 most curious interdependencies in the realm of Nature. Prof. 

 Hellriegel has discovered nodules on the rootlets of the plants, 

 tenanted by parasitic bacteria, which, while consuming a little of 

 the substance of the plant, j)ay a handsome rent in the compounds 

 of nitrogen which they build out of the air and pass to the fibers 

 that harbor them. These microscopic purveyors, when bred and 

 sown of set purpose, yield vastly increased crops of clover, alfalfa, 

 peas, cow peas, beans, and lupine. Of this abundant testimony 

 was presented at the Columbian Exhibition in the display of the 

 Experiment Stations in the Agricultural Building. 



Here, indeed, we come to the distinctive standpoint whence 

 knowledge sweeps its new horizons : its outlook upon Nature as a 

 whole, as a system intelligible only in the mutual interaction of 



VOL. SLV. 39 



