June 7, 1907] 



SCIENCE 



887 



it is necessary to use physiological means 

 of distinguishing, shall we say species? 

 For the present we can avoid making the 

 decision, and say forms or races, yet at the 

 same time we can hold our minds open to 

 evidence as to whether these forms or races 

 are not, after all, incipient species. Two 

 groups of these fungi especially force 

 themselves upon our attention from the 

 point of view we are considering. One of 

 the groups has been called the Uredinese 

 or rust fungi, and the ordinary rust of 

 cultivated cereals is a typical example. 

 The other group is known commonly as 

 the mildews, or more technically the family 

 Erisyphaceae. The rose mildew and the 

 grape mildew are common examples. In 

 both of these groups it has been found 

 necessary to distinguish what have been 

 called biologic forms or races, distin- 

 guished from each other only by the fact 

 that they differ in capacity to infect dif- 

 ferent species or genera of the host plant. 

 "Working with the wheat rust, which was 

 formerly supposed to be the same on any 

 of the cultivated cereals and wild grasses, 

 Eriksson has found that there are numer- 

 ous races adapted more or less closely to 

 the species of single genera, and they are 

 able to infect species of other genera either 

 with difBeulty or not at all. Their forms 

 can not be distinguished morphologically, 

 and yet the infection experiments show 

 that physiologically they are decidedly dif- 

 ferent from each other. In trying to con- 

 ceive the origin of these forms, there seem 

 to be three possibilities. First, these bio- 

 logic forms may have had an origin from 

 different species growing on a narrowly 

 limited group of host plants. There seems 

 to be little evidence for this view. Second, 

 they may have been derived from one 

 species, by sudden physiological changes 

 in the fungus alone, without any influence 

 of the host. This would be similar to the 

 origin of elementary species by mutation. 



as found by de Vries among the evening 

 primroses. There seems to be no direct 

 evidence for this view. Third, a group of 

 biological forms which can not be distin- 

 guished morphologically may have origi- 

 nated from one species which at first grew 

 on a wide range of host plants, but when 

 a strain or race is propagated continuously 

 on the same species of host, there is a spe- 

 cial adaptation of the fungus to that spe- 

 cies of host, and it becomes able to infect 

 that one more readily, and others less 

 readily, and at last not at all. For this" 

 view there is some direct evidence. A 

 form of rust which was capable of growing 

 on four genera of host plants, was propa- 

 gated for ten years continuously on only 

 one of the four. At the end of the ten- 

 year period it could infect that one genus 

 strongly and the other three weakly or 

 with uncertainty. If this experiment indi- 

 cates the way in which the biological forms 

 have come into existence, they have origi- 

 nated, not by mutation, but by adaptation. 

 The differences they exhibit have come 

 about by the gradual accumulation of im- 

 perceptible modifications. 



Among the mildews there has been found 

 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 adapta- 

 tion carried much farther than it is carried 

 among the rusts. 



The question remains, can these biologic 

 forms or adaptive races ever rise to the 

 dignity of true species? Again the direct 

 evidence is lacking. But if these fungi are 

 as variable in their morphological char- 

 acter as Klebs found even the flowering 

 plants to be under different physiological 

 conditions, we might expect the same 

 causes which bring about the physiological 



