December 7, 1900.] 



SCIENCE. 



879 



The only novel proposition employed in 

 this hypothetical solution of the problem is 

 that of the indefinite propagation of the 

 pathological efifects of a toxic compound of 

 animal origin, introduced in an infinitesimal 

 quantity and of so mild a physiological ac- 

 tion as to cause no immediately appreciable 

 damage. And yet the fact of inoculation 

 shows that this requirement must be ad^ 

 mitted as far as the internal phenomena are 

 concerned, whatever be the supposed nature 

 of the exciting cause. As already pointed 

 out by Dr. Smith, this phase of the subject 

 has analogy with other abnormal condi- 

 tions such as variegation of foliage, which 

 in some plants may also be propagated by 

 grafting. But albinism may easily prove to ^ 

 be a composite phenomenon, sometimes con- 

 stitutional and sometimes induced, and then 

 transmissible. That the albinism theory 

 of peach-yellows is not, however, necessarily 

 inconsistent with that of its origin through 

 mites is indicated by a curious fact for 

 which I am indebted to Mr. H. J. Webber. 

 An albinistic variety of orange, ' Drake's 

 Star,' was found in Florida to be entirely im- 

 mune to the orange-rust, caused by one of 

 the Phytoptidse. As albinism, or variega- 

 tion, is generally admitted to be connected 

 with or accompanied by a lack of vigor, 

 there is the greater warrant for expecting 

 that the present instance of immunity will 

 notprove to be due to any ability to resist the 

 attacks of the mites, but will be found ex- 

 plainable by the absence from the albinistic 

 protoplasm of the compounds affected by 

 the secretions of the Phytopti. A definite 

 relation of incompatibility having been es- 

 tablished between an albinistic condition 

 and a disease caused by Phytoptus, and in- 

 order of peach nursery stock has also been traced to 

 a mite, even though it may be found that the an- 

 imal is of a species difierent from that of the yellows. 

 Neither can we as yet exclude the possibility that the 

 peach may be less susceptible when young than after 

 it has reached bearing age. 



volving a yellow discoloration of the cells, 

 the physiological analogy between yellows 

 and albinism is strengthened, and the 

 anomalous proposition of an animal poison 

 causing a constitutional plant disease trans- 

 missible by budding loses something of its 

 apparent improbability. Further possibil- 

 ities not unworthy of mention lie in the 

 production of immunity in the peach 

 through inoculation with a less harmful 

 mite, or with a modified enzyme or antitox- 

 ine, or with a mild form of albinism, or 

 through the propagation of albinistic varie- 

 ties. The uniform susceptibility of the sorts 

 now in cultivation in the infested districts 

 renders it extremely improbable that a nor- 

 mal variety able to resist the yellows could 

 be secured by selection or introduction from 

 abroad. 



A further biological reason for belief in a 

 definite external cause for peach-yellows is 

 to be found in what may well be considered 

 a second species of the same genus, the so- 

 called ' peach-rosette,' which has appeared 

 in the orchards of Georgia and as far west 

 as Kansas. This disorder is of a much 

 more virulent type than the yellows, and is 

 able to attack and destroy the plum. It 

 has distinctly different, though similar, 

 symptoms and is likewise contagious and 

 communicable by budding. With two such 

 diseases occupying reasonable definite geo- 

 graphic areas and both repeatedly occurring 

 sporadically within wide general limits, the 

 probability is greatly increased that we are 

 dealing with the injuries of two species 

 of Phytoptus or related mites which are 

 normally parasitic on native vegetation, 

 probably indigenous species of Prunus, 

 whence they are accidentally transferred to 

 the peach and related fruits of foreign 

 origin, supposably by bees or by birds. 

 Such a method would explain the many 

 independent reappearances of the diseases, 

 and their capricious distribution in or- 

 chards, where contiguity or even contact 



