MICHIGAN ACADEMY OF SCIENCE. 39 



fact may be used in determining the nearness or remoteness of the relation- 

 ship between forms. This possibility has been recognized by many inves- 

 tigators and biologists have proposed using the degree of fertilit}' of hybrids 

 as the means of distinguishing genera, species, and varieties. Though this 

 has been founfl not to be relial)le in all cases, de Vries has suggested it as a 

 means of distinguishing his elementary species from varieties. If on crossing- 

 two forms the resulting hybrid is constant in regard to a given character, 

 when guarded against further crossing, the two forms were different species. 

 But if, on crossing, the descendants of the resulting piybrid follows Mendel's 

 law of hybrids according to which one-fourth of the offspring of the hybrid 

 in each succeeding generation resembles one parent in respect to a given 

 character, one-fourth resembles the other parent as regards the correspond- 

 ing character, while half are like the original hybrid, then the parent forms of 

 the hybrid were one and the same species. 



Whatever the limitations of this method in its practical application, the 

 significant fact is the extent to which physiological conceptions have in- 

 vaded a realm that was purely morphology. We may use the experimental 

 method in studying the origin of new species and varieties. We may apply 

 physiological methods in determining the range of the fluctuating variations 

 within the species. We may use physiological affinities as the test of the 

 degree 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 characteristics 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 a basis of species (hstinctions. The most important 

 relations which the l)acteria bear to the organic world in general, and to the 

 human race in particular are physiological in their nature. Some of them 

 have the power of invading the animal body and producing there substances 

 which we call toxins, and which may be so exceetlingly poisonous that the 

 result may be fatal in an extraordinarily short time. Fortunately the ani- 

 mal 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 lock-jaw. Also the same organism shows different powers of re- 

 sistance, or iramunit}^ at different periods. It is w^ell known that any con- 

 ditions of life that produce a low state of vitality in a given individual, make 

 that individual far more susce]3tible 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 neces- 

 sity of the so-called rotation of crops of higher plants is more dependent 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 plant that produced them. The question of the physiological varia- 



