42 NINTH REPORT. 



This capacity for ph3'siologicaI variation or adaptation on the part of 

 fungi is significant in another direction. It is certain that among the fungi 

 as well as among the bacteria, forms that for the most part live only on dead 

 organic matter, that is as saprophytes, may under certain special conditions 

 become adapted to a parasitic life. Thi^y thus become the producers of 

 new diseases. Though for the most part supposed new diseases are only 

 a wider distribution of old diseases, it is entirely possible for new dis- 

 eases actually to originate by physiological adaptation. This has been 

 proved in the production of plant disease experimentally. 



But if this kind of variation has its somber side, there. is also an obverse 

 side. Physiological variation enables us in many cuses to select and propa- 

 gate cultivated plants that are particularly resistant, and sometimes com- 

 pletely immune, to a given disease. The same phenomenon may be ob- 

 served here as in the'human family. In any given epidemic there are always 

 certain individuals who never contract the disease. They have a certain 

 natural immunity to that particular disease, and this immunity is 

 due to some physiological peculiarity. So in a field of rusted or mildewed 

 wheat some individual plants show themselves more resistant than their 

 fellows to the species of rust fungus found upon that species of host. By 

 selecting and propagating these immune individuals we may develop an im- 

 mune race or strain. The problem is not always so simple as here stated. 

 It may happen that a race immune to one disease may be very susceptible 

 to another, or immunity may be accompanied by other qualities altogether 

 midesirable. One might be led to suppose on reading certain popular ar- 

 ticles intended to show how new forms of plants are produced that it is only 

 necessary to imagine an ideal plant and then set to work to create it. Noth- 

 ing is farther from the truth than this. Nature does sometimes produce 

 something new, as a stoneless ])lum, or a nectarine on a peach tree. But 

 man can only take the materials furnished l)y natiu'e, combine them in new 

 ways, or modify them within limits which are usually soon reached.. He can- 

 not create a wheat plant immune to rust, nor a watermelon resistant to the 

 wilt fungus. But if nature furnishes a few individuals with the desired 

 f[ualities, man can propagate the individuals possessing these (pialities, and 

 by rigid selection maintain the qualities to a high degree. If it is possible 

 to cross the plants with other species or with varieties of the same species, 

 he may be able to combine in the same individual a number of desirable 

 qualities. Having obtained these qualities in one individual, he can best 

 conserve them by vegetative propagation, such as by grafts, cuttings, bulbs, 

 or tubers, according to the habit of the plant propagated. He may 

 care nothing whatever about the limits of species or varieties except in so 

 far as their physiological relations help or hinder his combinations. Fol- 

 lowing Mac Dougal's method it may be possible to produce in plants some 

 new characters. But even if it be possible to produce in this way really neAv 

 species, it is hardly within the range of possibility that we could choose l^e- 

 forehand the kind of a species we would produce. It would be a case of 

 "cut and try." If the result be a form with desirable qualities, preserve it, 

 but if it be worthless let it die. Nature has repeated this experiment ten 

 thousand times. If we would imitate her we must search out her secrets 

 in the physiological realm. She conceals them well, but is not unwilling 

 to reveal them to him who cjuestions her with a hearing ear, a seeing eye, 

 and a thinking brain, tools which she herself has given him. 



University of Michigan. 



