886 



SCIENCE 



[N. S. Vol. XXV. No. 649 



of relationship existing between different 

 forms found in nature. 



The foregoing discussion has had special 

 reference to the higher plants. But among 

 the lower forms of plant life physiological 

 methods are far more applicable, indeed 

 necessary, in determining the character- 

 istics of species. In all that group of 

 plants known as bacteria, species can be 

 distinguished only by physiological means. 

 These organisms are so simple in structure, 

 their morphological characters are so few, 

 it is utterly impossible to classify our 

 knowledge of them even from a systematic 

 point of view without using physiological 

 means as the basis of species distinctions. 

 The most important relations which the 

 bacteria bear to the organic world in gen- 

 eral, and to the human race in particular, 

 are physiological in their nature. Some of 

 them have the power of invading the ani- 

 mal body and producing there substances 

 which we call toxins, and which may be so 

 exceedingly poisonous that the result may 

 be fatal in an extraordinarily short time. 

 Fortunately the animal body has the power 

 to vary its ordinary physiological processes 

 in such a way as to produce antitoxins 

 which neutralize the action of the toxins. 

 A given organism may vary in its virulence 

 at different times. An epidemic due to 

 an organism in the so-called attenuated 

 state, produces a mild form of the disease. 

 A given animal or plant may be especially 

 resistant to the toxin of one species of 

 bacteria, as the horse is to diphtheria toxin, 

 or it may be very susceptible to a given 

 toxin, as the human body is to the toxin of 

 tetanus, or lockjaw. Also the same organ- 

 ism shows different powers of resistance, or 

 immunity, at different periods. It is weU 

 known that any conditions of life that pro- 

 duce a low state of vitality in a given 

 individual, make that individual far more 

 susceptible to disease, that is, to the toxins 

 of other organisms. Not only are plants 



and animals susceptible to the toxins pro- 

 duced by other plants and animals, but 

 each organism produces substances which 

 are toxic to itself. This is true not only 

 for the lower organisms, but at the present 

 time a discussion is being carried on as to 

 whether the necessity of the so-called rota- 

 tion of crops of higher plants is more de- 

 pendent upon the partial exhaustion of the 

 soil in elements necessary for a given crop, 

 or upon the gradual accumulation in the 

 soil of substances detrimental to the kind 

 of plants that produced them. The phys- 

 iological variations of organism, and the 

 physiological relations of one kind of 

 organism to another, form a series of the 

 most fascinating as weU as the most dif- 

 ficult of biological problems. The smaU 

 size of the bacteria and the rapidity with 

 which they multiply make them very 

 favorable subjects for experiment along the 

 line of the fundamental biological proc- 

 esses. An organism that requires several 

 hundred years to complete its life cycle 

 is obviously not a favorable subject for an 

 experiment that requires the study of sev- 

 eral generations. But if, as in the case 

 of some of the bacteria, a new generation 

 may be produced every fifteen minutes, it 

 is possible to obtain within a few hours 

 hundreds of generations and millions of 

 individuals. 



There is another group of organisms 

 about which I wish to speak, not so simple 

 as the bacteria in structure, but far in- 

 ferior in that respect to the highest plants. 

 I refer to the filamentous fungi, and I 

 wish to call your attention to some facts 

 that again have to do with the question: 

 What is a species? As in the case of 

 higher plants, the first attempts to classify 

 these organisms were upon a purely mor- 

 phological or structural basis. But a deeper 

 knowledge of their life histories and phys- 

 iological variations makes it more and more 

 apparent that here, as among the bacteria,. 



