10 MR. A. R. WALLACE ON THE PAPILIONIDÆ 
albinoism and melanism may, as I have already stated, be considered as anälogous facts; 
and I met with one case of a bird, a species of Lory (Eos fuscata, Blyth), clearly existing 
under two forms, since I obtained both sexes of each from a single flock. 
The fact of the two sexes of one species differing very considerably is so common, that 
it attracted but little attention till Mr. Darwin showed how it could in many cases be 
explained by what he termed sexual selection. For instance, in most polygamous animals 
the males fight for the possession of the females, and the victors, always becoming the 
progenitors of the succeeding generation, impress upon their male offspring their own 
superior size, strength, or unusually developed offensive weapons. It is thus that we can 
account for the spurs and the superior strength and size of the males in Gallinaceous 
birds, and also for the large canine tusks in the males of fruit-eating Apes. So the 
superior beauty of plumage and special adornments of the males of so many birds 
can be explained by supposing (what there are many facts to prove) that the females 
prefer the most beautiful and perfect-plumaged males, and that thus slight accidental varia- 
tions of form and colour have been accumulated till they have produced the wonderful 
train of the Peacock and the gorgeous plumage of the Bird of Paradise. Both these 
causes have no doubt acted partially in insects, so many species possessing horns and power- 
ful jaws in the male sex only, and still more frequently the males alone rejoicing in rich 
colours or sparkling lustre. But there is here another cause which has led to sexual 
differences, viz. a special adaptation of the sexes to diverse habits or modes of life. This 
is well seen in female Butterflies (which are generally weaker and of slower flight), often 
having colours better adapted to concealment; and in certain South American species 
(Papilio torquatus) the females, which inhabit the forests, resemble the Æneas group, 
which abound in similar localities, while the males, which frequent the sunny open river- 
banks, have a totally different coloration. In these cases, therefore, natural selection 
seems to have acted independently of sexual selection ; and all such cases may be con- 
sidered as examples of the simplest dimorphism, since the offspring never offer interme- 
diate varieties between the parent forms. 
The distinctive character therefore of dimorphism is this, that the union of these dis- 
tinct forms does not produce intermediate varieties, but reproduces them unchanged. 
In simple varieties, on the other hand, as well as when distinct local forms or distinct 
species are crossed, the offspring never resembles either parent exactly, but is more or 
less intermediate between them. Dimorphism is thus seen to be a specialized result of 
variation, by which new physiological phenomena have been developed ; the two should 
therefore, whenever possible, be kept separate *. 
3. Local form, or variety.—This is the first step in the transition from variety to species. 
* The phenomena of dimorphism and 
: polymorphism may be well illustrated b : j 
haired Saxon man had two wives, : ed by supposing that a blue-eyed, flaxen 
one a black-haired, red-skinned Indian squaw, the other a woolly-h N 
l > ^ y-headed, sooty 
vor wis t that instead of the children being mulattoes of brown or dusky tints, mingling the separate 
characteristics of their parents in varying degrees, all the boys should b ike thei i 
iiis vould dissi NR one se je Fa y e pure Saxon boys like their father, while the 
exactly like her fellow-wife, and altogether differing 
human beings having similar physiological 
* 
ag nai od ies V7 UE Lis TN MÀ 
when 
