€92 ' REPORT— 1894. 



preserved types in our inland ponds and lakes, types such as the housefly and cock- 

 roach, which for ages have remained the same, illustrate this stability. Living 

 matter must therefore be capable both of the power to vary and of stability ; tho 

 jirst it possesses, the second it gains by sexual conjugation, which tends to prevent 

 the slight deviations of a form which has become adapted to its environment from 

 producing still further deviation by blending them together so that some, at any 

 rate, of their progeny may preserve the useful ancestral qualities. To sex we owe 

 our fairly defined species and genera ; without sex we cannot doubt that life would 

 exist in the form of innumerable varieties that we should fail to group together. 



6. On the Relation of Mimetic Characters to the Original Form. 

 By F, A. DiXEY, M.A., M.D., Fellow of Wadham College, Oxford. 



An objection that has been often brought against the theory of mimicry, as 

 enunciated by Bates and accepted by Darwin, is the difficulty of imagining th© 

 first stages in the production of a mimetic pattern. Fritz Miiller ' endeavoured to 

 meet this objection by alleging that mimicry chiefly originated between forms that 

 already bore considerable resemblance to each other. The main instance (that of 

 Leptalis melia) on which he relied in order to prove his point was not well chosen, 

 for there is reason to think that he was in error both in considering that it 

 represented the ancestral form of Leptalis and in supposing that it was not 

 protected by mimicry. Nevertheless, his contention is sound in so far as it 

 emphasises the fact that the process of mimetic assimilation depends rather on the 

 development of old than on the starting of new features. 



An illustration of this principle is afforded by a comparison of the non- 

 mimetic butterflies Pieris locusta and P. thaloe with the mimetic species of the 

 closely allied genus Mylothris, and with Heliconius numata, which serves as the 

 model for the latter ; all these forms inhabiting the same part of the neotropical 

 region. An almost perfect transition can be traced on the undersides from the 

 non-mimetic species of Pieris, through M. li/pera $ , M. lorena $ , M. pyrrha $ , 

 M. lorena J , to M. pyrrha $ , this last butterfly being a very close copy of 

 Heliconius numata. The whole series shows (1) that a practically perfect 

 mimetic pattern can be evolved by gradual and easy stages without any violence 

 or abruptness of change ; (2) that it is not necessary that the forms between 

 which mimicry originates should possess considerable initial resemblance ; (3) that 

 80 small a beginning as the basal red patches on the underside of the hind-wing of 

 many Pierines gives eufiicient material for the assimilative process to work on. 



The feebler development of the mimetic pattern in the males of this group calls 

 for some explanation. No doubt the females require more protection, but does 

 there exist any active check on the fuller assumption of mimetic patterns by the 

 males P The retention of the original white by the latter sex has been in similar 

 instances attributed to female choice ; Mr. Wallace, on the other hand, thinks it 

 due to the difference of habits in the two sexes, the females alone flying in 

 company with the mimicked Heliconii. But this leaves unexplained the presence 

 of a partial mimetic pattern in the male. The probability is that, although on the 

 •wing it may be advantageous rather than otherwise to the male, as Mr, Wallace 

 thinks, to be taken for an ordinary white butterfly, yet when the insect is at rest and 

 settled with the wings erect, any Heliconine resemblance would be to some extents 

 protective; and the whole aspect of these males, the underside alone of which shows 

 any mimetic features, is the resultant of these two divergent tendencies. The mimetic 

 features of the male cannot be regarded as a mere incidental result of the more com- 

 plete transformation of the female, because in many species of other groups the female 

 is completely mimetic while the male shows no approach whatever to a mimetic 

 change ; moreover, there is a species of Hesperocharis (IL hirlanda) in which not 

 only the male but both sexes show a partial mimetic pattern no further advanced 

 than that of M. lorena $ or M. pyrrha ^. It is difiicult to believe that in thia 

 case the pattern is not in some degree protective. 



' Jenaitch. Zeitschr., vol. x. 



