62 



THE YOUNG 



NATURALIST. 



VARIATION 

 IN LEPIDOPTERA. 



REVERSION. 



By John E. Robson. 



It may be necessary to explain the mean- 

 ing of some words such as reversion, used in 

 these papers. Reversion is a term used to 

 express a well-known principle, though the 

 causes of it are not easy to explain. When 

 animals under domestication or other 

 changed circumstances, assume an appear- 

 ance differing more or less from their 

 original form, it frequently occurs that some 

 characteristic of the parent species, which 

 has apparently been lost among the changes 

 that have been made, re-appears after many 

 generations. This is most noticeable under 

 domestication, for we are rarely, if ever, 

 able to point to two animals in a state of 

 nature, of distinctly different appearance, 

 and show that one was an actual descendant 

 of the other. Mr. Darwin gives some very 

 striking illustrations in his works, and one 

 in particular in The Origin of Species may be 

 referred to here. Mr. Darwin took pigeons 

 of three distinct races, all of which were 

 known to " breed true " — that is, to produce 

 offspring like themselves. He paired a white 

 Fantail with a black Barb, and a black 

 Barb with a Spot (a white bird with red 

 tail and a red spot on the head). He then 

 paired their progeny and from them pro- 

 duced a blue bird, with the black bars on 

 on the wings, black-barred and white-edged 

 tail, and other peculiarities of the original 

 Rock Dove, from which all our races of 

 domestic pigeons are descended. None of 

 the birds crossed had any blue colour about 

 them, none of them had the wing or tail 

 bars, yet the young of the second crossing 

 reverted back to the original colouring and 

 marking of the bird from which they had 

 sprung many hundreds, perhaps thousands 

 of generations before. This then is called 



reversion, and though we may rarely be able 

 as I have said, to point to two different 

 animals in a state of nature, and show that 

 one has actually descended from the other, 

 we may nevertheless be able to recognise 

 characteristics that have been derived from 

 some common ancestor. All animals of the 

 same genus, have certain characteristics in 

 common, probably derived from an ancestor 

 in the (comparitively) not very remote past. 

 All animals of the same family have charac- 

 teristics also derived from a common ances- 

 tor, but as they have diverged more from the 

 ancestral form, we are safe in assuming that 

 the ancestor was considerably more remote 

 than in the case of the ancestor of the genus. 

 The possession of characters in common is, 

 of course, not any positive proof of descent 

 from a common ancestor, but it is clear that 

 the greater the number of characters pos- 

 sessed in common by two or more species, 

 the greater is the probability of such 

 descent ; and the larger the number of 

 species possessing a common character, the 

 less is it likely that it has been acquired 

 independently. The Geometry may be 

 taken as an illustration. We have in Britain 

 above 300 species of this group, many of 

 them have a distinct dark band across the 

 central part of the forewing, and still more 

 of them have the outlines of such a band, 

 a costal blotch where the band should 

 begin, or some other trace of it. In fact 

 there are few species indeed that have not 

 some markings connected with the band. 

 If this character appeared only in one or 

 two species, or one or two closely allied 

 genera, it might be assumed that the ances- 

 tor from which such bar was derived, was 

 not one of most remote antiquity. But 

 when this characteristic, or some trace of it, 

 is found in most of the species, as well in 

 very distinct genera, as in others more 

 closely allied, it carries us back to a very far 

 distant ancestor indeed. Confining our- 

 selves to British species, we may easily 



