310 Mr. A. R. Wallace on 



marked and coloured almost exactly as in Thyca hclladonnci, and 

 the female resembles the same insect almost as much on the 

 upper surface ; both are found at Darjeeling. P'ler'is s'llana, a 

 species peculiar to Ceylon, wonderfully resembles on the under- 

 side the common Thyca cucharis which ranges all over India. Pr'io- 

 neris cornelia of Borneo, as figured by Vollenhoven, is so like 

 Thyca singhapura that I at first thought it might be a variety of 

 that species till I found by the description that it was structurally 

 different. Lastly, Pieris Icela of Timor departs so much from the 

 style of colouring of its allies and approaches so nearly to tliat of 

 Thyca belisama of Java, that I should almost look for an ally of 

 the last species to be discovered in Timor to serve as its pattern. 

 We know so little of that island at present, and so many species 

 having the same distribution as T. belisama extend to Timor, or 

 are represented there by an allied form, that we might, quite in- 

 dependently of any connexion with P. Icela, expect such a species 

 to be found there. On the whole, then, I think we have sufii- 

 cient facts to show that phenomena of mimicry, small in amount 

 but of the same character with those that are so fully developed 

 in many other groups, do occur among the Pieridcs ; and this is 

 just what we might expect sometimes to meet with in nature, for 

 whenever a group of species possesses any amount of special pro- 

 tection, even if it only leads some few of their enemies to avoid 

 them, it will then become advantageous for the species of other 

 groups to be mistaken for these, and any accidental variations 

 that may tend to bring about a resemblance will generation after 

 generation survive, till further imitation ceases to be useful or 

 is overbalanced by some concurrent disadvantage. 



In a few cases what appears to be a dimorphic female occurs ; 

 the difference being however one of colour only.. In Tachyr'u 

 paulina, the female {neomho, Boisd.) is either white with a pearly 

 gloss beneath, or yellow with the underside rich ochre. In an 

 insect which I consider with some hesitation to be the female of 

 Tachyris celestina, a pure white and a rich yellow variety also 

 occur. In Tachyris cycinna both sexes are either greenish sul- 

 phur-yellow or rich chrome-orange. In Tachyris zarinda the 

 males are either clear reddish-orange or deep burnt crimson. In 

 none of these cases have intermediate forms ever been found, and 

 though we have no direct proof in any case that the two forms 

 are continually produced from a single pair of individuals, there is 

 every reason from analogy to suppose that such is the fact. It is 



