y>7Q Professor E. B. Poulton [March 5, 



range. A good example is furnished Ijy these same Fseudacrseas in 

 Uganda. The commonest form in the Entebbe district, as shown 

 by the prolonged researches of C. A. Wiggins, has been known as- 

 /. hobleyi, witli its male mimicking the male of Planema macaristay 

 as well as both sexes of P. poggei, its female, originally described as 

 Psendacraea tirikensis, mimicking the female of Planema macarista. 

 C. A. Wiggins' splendid material at Oxford shows that, a very small 

 proportion of the females of Psendacrsea hoUcyi resemble their males 

 and mimic the same models as these latter. Now, as we pass eastward 

 round the northern shores of the Victoria Xyanza, I^lanema macarista, 

 far commoner than P. poggei at Entebbe, becomes rarer and 

 apparently ceases on the western side of the Nile, issuing from the 

 lake at the Rippon Falls, near Jinga. P. poggei, on the other liand, 

 continues much farther eastward, to Mount Elgon and beyond it to 

 the north-eastern corner of the lake, and probably farther into 

 British East Africa. In correspondence with this distribution S. A. 

 Neave found that the male-like female of Pseudacraea hohJeyi 

 becomes the predominant form east of the Nile. On the other hand, 

 the female form tirikensis, resembling the female of Planema 

 macarista, is predominant where, as at Entebbe, its model is common^ 



Another interesting geographical change is witnessed when we 

 compare C. A. Wiggins' experience at Entebbe with &. I). H. 

 Carpenters in Damba and Bugalla Islands in the lake. At Entebbe 

 the above-mentioned Pseudacraea Jwbleyi, with sexes nearly always 

 unhke, is accompanied by two other mimetic forms with sexes alike, 

 all three, as we know from Carpenter's researches, belonging to the 

 same species. These are terra, mimicking Planema tellus, and 

 ohscvra, mimicking P. paragea. At Entebbe the total number of these 

 forms together is shown by Wiggins' investigations to be about one- 

 third of their Planema models. On the two islands the proportions 

 are very different, the models being greatly outnumbered by their 

 mimics. On Bugalla, indeed, where the chief captures were made, 

 the Planemas were not much over one-third of the Pseudacrdeas. 

 We do not know^ the cause of this difference : it may be due to 

 difference in the abundance of the food-plants or in some other 

 condition. Corresponding with this reversal in the proportions, the 

 mimetic forms, which are on the whole constant and sharply separated 

 on the mainland, tend to vary and to run into each other on the 

 two islands. The mimicry keeps true where the models are relatively 

 abundant, but often tends to become blurred where they are rela- 

 tively scarce. It must also be borne in mind that even on the 

 mainland the Pseudacrseas are far commoner than we should expect 

 them to be, on the hypothesis of Bates. 



Passing from Entebbe to the westw\4rd, other interesting changes 

 are seen in both models and mimics. It has been explained that in 

 the Entebbe district, the male of one chief model, Planema macarista, 

 is mimicked by the male of Pseudacrsea Jiolleyi. Both are dark 



