On the islets of the little Madeira group there are many insects which are characterized as varieties in Mr.Wollaston's admirable work, but which would certainly be ranked as distinct species by many entomologists.

Even Ireland has a few animals, now generally regarded as varieties, but which have been ranked as species by some zoologists.

Several experienced ornithologists consider our British red grouse as only a strongly marked race of a Norwegian species, whereas the greater number rank it as an undoubted species peculiar to Great Britain.

A wide distance between the homes of two doubtful forms leads many naturalists to rank them as distinct species; but what distance, it has been well asked, will suffice if that between America and Europe is ample, will that between Europe and the Azores, or Madeira, or the Canaries, or between the several islets of these small archipelagos, be sufficient?
Mr.B.D.Walsh, a distinguished entomologist of the United States, has described what he calls Phytophagic varieties and Phytophagic species.
Most vegetable-feeding insects live on one kind of plant or on one group of plants; some feed indiscriminately on many kinds, but do not in consequence vary.

In several cases, however, insects found living on different plants, have been observed by Mr.Walsh to present in their larval or mature state, or in both states, slight, though constant differences in colour, size, or in the nature of their secretions.
In some instances the males alone, in other instances, both males and females, have been observed thus to differ in a slight degree.