36 SYMBIOGENES1S 



adequate symbiotic services. The food gotten by dishonest 

 living though many species have contracted the predaceous 

 habit and make dishonest gain their "usual food" and are 

 apparently flourishing is not permanently advantageous in 

 any real sense. Hence there arises in due time for purposes 

 of health and of sexual reproduction the requirement for a 

 change of food, a longing for the "simple life." A recovery 

 is made provided the bad habits were not carried to the extent 

 of "obligate" parasitism which means in-feeding so intense, 

 fastidiousness so morbid, and loss of the sense of proportion so 

 complete, that the way is barred to a return to a wholesome 

 diet. 



Both rose-aphis and C. roscoffensis show by their "urgent 

 need of a change of diet" that they have already entered in 

 varying degrees upon the path of retrogression " dragging 

 Evolution in the mud." That parthenogenesis as here 

 exemplified by the rose-aphis is a degenerate form of 

 reproduction has long been recognised. It is based on 

 the opposite methods of life to those which produce symbio- 

 genesis. When the rose-aphis flies off to certain grasses to 

 cease its career of indulgence on the rose and to feed instead on 

 them, this is in order to restore a former condition of symbiotic 

 normality by means of a tolerably normal mode of feeding 

 which permits the re-acquisition of the power of sexual 

 reproduction. We may take it that the aphis is not a pest on 

 these grasses to the same extent as it is on the rose, and by 

 being able to make the change it shows that it has not yet 

 become as fastidious as obligate parasites which cannot 

 change their host under penalty of death. The aphis thus 

 purchases rejuvenescence by curbing its anti-symbiotic pro- 

 pensities and giving up conditions of surfeit. That Convoluta 

 stands in like case we shall presently find. What similarities 

 there are between aphis and plantagen, therefore, are in the 

 last analysis similarities of instability and of a restoration of 

 some stability by similar means, i.e., by a return to modera- 

 tion and proper conditions of symbiosis. Mr. Davidson contents 



