MEMOIR. xliii 
“ This principle can be no other than natural selection, the select- 
ing agents being insectivorous animals, which gradually destroy 
those sports or varieties that are not sufficiently like Jthomie to 
deceive them. ... Such, I conceive, is the only way in which the 
origin of mimetic species can beexplained. I believe the case offers 
a most beautiful proof of the truth of the theory of natural selection. 
It also shows that a new adaptation, or the formation of a new 
species, is not effected by great and sudden change, but by nume- 
rous small steps of natural variation and selection.”* 
Two questions suggest themselves. 1. Why are the Helconide, 
which by their brilliant colouring attract the eye of insectivorous 
birds, and by their slowness on the wing are easily seized, mimicked 
more than other families of butterflies? and, 2. Why are insects so 
much more subjects of mimicry than other animals? 
Both questions are answered by Bates. 1. As there is nothing 
apparent in the structure or habits of the Helicontde which could 
render them safe from persecution by the numerous insectivorous 
animals which are ever on the watch in the same parts of the forest 
which they inhabit, “it is probable that they are unpalatable to 
insect enemies. Some of them have exsertile glands near the 
arms, which are protruded when the insects are roughly handled ; 
it is well known that similar organs in other families secrete fetid 
liquids or gases, and serve as a protection to the species. I never 
saw the flocks of slow-flying Helzconid@ in the woods persecuted by 
birds or dragon-flies, to which they would have been an easy prey ; 
nor, when at rest on leaves, did they appear to be molested by lizards 
or the predaceous flies of the family Asz/zde, which were very often 
seen pouncing on butterflies of other families. If they owe their 
flourishing existence to this cause, it would be intelligible why the 
Leptalide, whose scanty number of individuals reveals a_ less 
protected condition, should be disguised in their dress, and thus 
share their immunity.” fT 
There can be little doubt that this is the explanation. Mr. 
Wallace adduces like evidence as to these skunks of the insect world. 
He says that the families of butterflies, Helicontde, Danaid@, and 
Acreide—including a thousand species characteristic of the three 
tropical regions, South America, South Asia, and Africa—“all have 
ample, but rather weak wings, and fly slowly; are very abundant, 
conspicuous in colours and markings, never try to conceal them- 
* Trans. of Linn. Soc., pp. 512, 513. + Jozd., p. 510, and infra, p. 106. 
