MICHIGAN ACADEMY OF SCIENCE. 41 



could infect that one genus strongly and the other three weakly or with 

 uncertainty. If this experiment indicates the way in Avhich the biological 

 forms have come into existence, they have originated, not by mutation, but 

 by adaptation. The difference they exhibit have come about by the grad- 

 ual accumulation of imperceptible modifications. 



Among the mildews there has been foimd an adaptation of forms even 

 closer than among the rusts. Experiments of Salmon on the mildews of 

 grasses disclosed the fact that adaptation is not only to one or few genera, 

 but in many cases actually to one or a few species within the genus. The 

 mildews exhibit the phenomena of adaptation carried much farther than 

 it is carried among the rusts. 



The question remains, can these biologic forms or adaptive races ever rise 

 to the dignit}^ of true s])ecies. Again the direct evidence is lacking. But if 

 these fungi are as variable in their morphological character as Klebs formd 

 even the flowering plants to be under different physiological conditions, we 

 might expect the same causes which bring about the physiological adapta- 

 tion to be able to produce morphological differences as well. But even if 

 no morphological differences appear, are we not justified in making ph3'sio- 

 logical characters the basis of species among the fungi as is already done 

 among the bacteria? The speaker is inclined to answer this question 

 in the affirmative. It seems certain, that for practical ]iurposes at least, it 

 is becoming absolutely necessary in other groups of fungi as well as in the 

 rusts and mildews, to make distinctions on physiological grounds, not to 

 the exclusion of morphology, but in addition to it. Whether you call the 

 grou])s of individuals so distinguished species or not, matters very little. 

 The important thing is that the distinction must be made. 



It is impossible to apply de Vries' test for species and varieties among the 

 fungi. For most of them there can be no such thing as cross-fertilization. 

 For many there is no fertilization at all, and even where present it is gener- 

 ally strictly self-fertilization. Naegeli long ago pointed out that where 

 plants are propagated only vegetatively or by self-fertilization, and it may 

 be added parthenogenetically, individual peculiarities Avere perpetuated in 

 the descendants, while with open or cross-fertilization the peculiarities of 

 one individual may be modified in the next generation by mingling with an- 

 other line of inheritance representing peculiarities of another individual op- 

 posed to those of the first. Open or cross-fertilization therefore tends to 

 keep the species homogeneous by neutralizing extreme individual variations. 

 While in those plants which are propagated by parthenogenesis, that is where 

 the eggs develop without fertilization, or by self-fertilization, or by non- 

 sexual spores, or by vegetative means, the species tend to become hetero- 

 geneous. They are made up of many lines of descent which are never mingled, 

 individual peculiarities tend to become extreme, and species limits are par- 

 ticularly difficult to determine. Among flowering plants the hawkweeds 

 furnish an example of the results of reproduction by parthenogenesis. In this 

 genus, Hieracium, it is said that of tAvo noted men Avho had made special 

 study of the species of the genus, neither could identify the species by the 

 other's descriptions. The same result is apparent among the fungi, in the 

 development of the l^iologic forms or adaptiA-e races. Individual adaptation 

 to a giA'en host is not neutralized by fertilization from a plant Avith a diff'er- 

 ent adaptation, but is continually accentuated. The practical importance 

 of many of these adapted forms compels us to recognize them as distinct en- 

 tities, and to giA^e them names. For practical purposes then they are species, 

 e\^en though they can be distinguished only ph3'siologically. 

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