The venation of the wings of insects is another example of modifications without serving any special purpose. There is no vein in certain Thripidae, and only a rudiment or a single vein in Chalcididae. There are thousands of variations more or less marked, some of the same type with comparatively trivial variation, others presenting distinct types, even in the same family, such genera, for example, as _Polyneura_, _Tettigetra_, _Huechys_, &c. in the Cicadidae.
Individual differences have often been regarded as distinctive of species; varieties also are very deceptive, and races come very near to species. A South-American beetle, _Arescus histrio_, has varieties of yellow, red, and black, or these colours variously intermixed, and, what is very unusual, longitudinal stripes in some and transverse bars in others, and all taken in the same locality. Mr. A. G. Butler, of the British Museum, is of opinion that 'what is generally understood by the term species (that is to say, a well-defined, distinct, and constant type, having no near allies) is non-existent in the Lepidoptera, and that the nearest approach to it in this order is a constant, though but slightly differing, rare or local form--that genera, in fact, consist wholly of a gradational series of such forms (Ann. Mag. Nat. Hist. 5, xix. 103)[127].'"
[127] Pascoe, _The Darwinian Theory of the Origin of Species_, 1891, pp. 31-33, and 46.
So much as regards entomology, and still living forms. In illustration of the same principles in connexion with palaeontological series, I may quote Würtenberger, who says:--
"With respect to these fossil forms [i.e. multitudinous forms of fossil Ammonites], it is quite immaterial whether a very short or a somewhat longer part of any branch be dignified with a separate name, and regarded as a species. The prickly Ammonites, classed under the designation of Armata, are so intimately connected that it becomes impossible to separate the accepted species sharply from one another. The same remark applies to the group of which the manifold forms are distinguished by their ribbed shells, and are called Planulata[128]."
[128] _Neuer Beitrag zum geologischen Beweis der Darwinischen Theorie_, 1873.