An Instance [of Mutation. 147 



lelism between C. citricola and C. hesperidum, on the one 

 hand, and P. psidii and C viridis, and C. viridis and C r . 

 colemani, on the other,, will now be obvious. There is one 

 apparent difficulty. Citricola is a species described in 

 I '.ML. but hesperidum was described many years earlier. 

 According to my hypothesis citricola should be regarded 

 as the parent species and as having been earlier in time. 

 It has, however, to be remembered that the mere fact of 

 an earlier record is of itself insufficient to prove the later 

 origin of a species. What has probably happened is that 

 hesperidum formed out of citricola crowded out the parent 

 species, and this would fit in exactly with my hypothesis 

 and with the observations in Mysore and other countries 

 where Pulvinaria psidii is seldom a pest, while C. viridis 

 and C. colemani are notoriously injurious to crops. 



The second instance is that which has come to the notice 

 of Green, and to which he refers in the course of an in- 

 teresting letter received from him on the subject of the 

 phenomenon in C. viridis, brought to his notice. He 

 wrote as follows : " Such degeneration, if clearly estab- 

 lished, is extremely interesting, and so far as I know has 

 not been recorded before. Curiously enough since reading 

 this paper I have met with an instance that appears to be 

 of a similar nature. In examining some old material 

 from Java, I have found an insect that agrees in every 

 character with Phenaeoccns mangiferae, described from 

 Ceylon, except that its antennae have only seven instead 

 of nine joints. According to the present classification, 

 this difference would necessitate the relegation of the 

 Javan specimens to a distinct genus (Pseudococcus). But 

 I am convinced that they are really conspecific." 



What has undoubtedly occurred in C. viridis is therefore 

 by no means an isolated instance, and I believe the study 

 of scale insects in the light of the phenomenon recorded 

 and described above will bring to light more instances of 

 mutation. We are still too ignorant of the obscure pro- 

 cesses involved in this important phenomenon to neglect 

 what seems to me to be a promising field for its investi- 

 gation. 



In conclusion, I have to thank Dr. Coleman, the Director 

 of Agriculture in Mysore, for his sympathy and guidance, 

 and through him Prof. Keuchenius of Java, Mr. C. C. 

 Gowdey of Uganda, Mr. Lyne, Director of Agriculture in 

 Ceylon, Mr. P. R. Dupont, Curator, Botanic Station, 



