SCIENCE 



[N. S. Vol. XXV. No. 619 



adaption to be able to produce morpho- 

 logical differences as well. But even if no 

 morphological differences appear, are we 

 not justified in making physiological char- 

 acters 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 cer- 

 tain that, for practical purposes at least, 

 it is becoming absolutely necessary in other 

 groups of fungi, as well as in the rusts and 

 mildews, to make distinctions on physi- 

 ological grounds, not to the exclusion of 

 morphology, but in addition to it. 

 Whether you call the groups of individuals 

 so distinguished species or not, matters 

 v€ry little. The important thing is that 

 the distinction must be made. 



It is impossible to apply de Vries's 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 generally strictly self- 

 fertilization. Naegeli long ago pointed out 

 that where plants were propagated only 

 vegetatively or by self-fertilization and it 

 may be added parthenogenetically, indi- 

 vidual peculiarities were perpetuated in 

 the descendants, while with open or cross- 

 fertilization the peculiarities of one indi- 

 vidual may be modified in the next gen- 

 eration by mingling with another line of 

 inheritance representing peculiarities of 

 another individual opposed 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 propa- 

 gated 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 heterogeneous. They are 

 made up of many lines of descent which 

 are never mingled, individual peculiari- 



ties tend to become extreme, and species 

 limits are particularly difficult to deter- 

 mine. Among flowering plants the hawk- 

 weeds furnish an example of the results of 

 reproduction by parthenogenesis. In this 

 genus, Hieracium, it is said that of two 

 noted men who had made a special study 

 of the species of this genus, neither could 

 identify the species by the other's descrip- 

 tions. The same result is apparent among 

 the fungi, in the development of the bio- 

 logic forms or adaptive races. 



Individual adaptation to a given host is 

 not neutralized by fertilization from a 

 plant with a different adaptation, but is 

 continually accentuated. The practical 

 importance of many of these adapted 

 forms compels us to recognize them as dis- 

 tinct entities, and to give them names. 

 For practical purposes then they are spe- 

 cies, even though they can be distinguished 

 only physiologically. 



This capacity for physiological variation 

 or adaptation on the part of fungi is signi- 

 ficant 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. They 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 diseases actually to origi- 

 nate 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 

 cases to select and propagate cultivated 

 plants that are particularly resistant, and 

 sometimes completely immune, to a given 

 disease. The same phenomenon may be 

 observed here as in the human family. In 



