An Instance of Mutation. 1 l-~> 



The tendency to regressive mutation exhibited by 

 these insects is probably due to the continued absence of 

 a sexual generation, which, if one may judge from the 

 behaviour of species similarly circumstanced, tells on the 

 vigour and vitality of the species. Though C. hesperidum 

 is one of the commonest species occurring on numerous 

 plants from the United States to Japan, no male has been 

 recorded at any rate from India, Java or Ceylon ; nor 

 have males been recorded for C. viridis, the study of which 

 dates as far back as 1882, except for two doubtful ones 

 from Java. 



The Weismanian theory that the purpose of sexual 

 reproduction is to induce variability has received no 

 support from the study of variation in parthenogenetic 

 forms, the results of which show that variability in such 

 species is not less than that in sexually produced forms, 

 and that therefore variability is not a factor necessarily 

 introduced by the union of the sexes. But from the fact 

 that parthenogenesis does not induce variability it does not 

 follow that it is the cause of it. I suggest it as a possibility 

 because the types I have been dealing with show a pro- 

 gressive degeneration, and because it seems to me that 

 the continued absence of a male generation prevents the 

 swamping effects of intercrossing, and therefore affords a 

 greater chance for the survival of variations. Whether or 

 not the continued absence of a sexual generation is the real 

 explanation of the instability of P. psidii, it is the sort of 

 species where one would look tor mutation. Much the 

 same remarks apply to C. viridis, which take so many 

 different forms in different countries. There is thus con- 

 siderable justification apart from the facts I have already 

 given for the conclusion that C. viridis, C. colemani, C. 

 africanum, and the Javan forms are directly or indirectly 

 derived from P. psidii. 



This conclusion is of great importance and interest. It 

 indicates that the parallelism in structure betw r een genera 

 with ovisacs and those without them have an evolution- 

 ary connection, the ovisac condition being antecedent in 

 time. Such genera could be found in families other than 

 Lecaniinae. In Dactylopinae, for instance, there is a 

 structural similarity between one oviparous and another 

 viviparous species in Mysore. In Pulvinaria itself, there 

 are probablv other species which stand to species in Le- 

 canium in the same relation as psidii does to viridis. In 



TRANS. ENT. SOG. LOND. 1918. — FART.S I. II. (DEC.) L 



