DOUBTFUL SPECIES. 43 



from the American mainland, I was much struck bow en- 

 tirely vague and arbitrary is the distinction between species 

 and varieties. 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 cer- 

 tainly 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 Nor- 

 wegian species, whereas the greater number rank it as an 

 undoubted species peculiar to Great Britain. A wide dis- 

 tance between the homes of two doubtful forms leads many 

 naturalists to rank them as distinct species ; but what dis- 

 tance, 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 sev- 

 eral islets of these small archipelagoes, 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 color, 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. When the differ- 

 ences are rather more strongly marked, and when both sexes 

 and all ages are affected, the forms are ranked by all ento- 

 mologists as good species. But no observer can determine 

 for another, even if he can do so for himself, which of these 

 Phytophagic forms ought to be called species and which 

 varieties. Mr. W T alsh ranks the forms which it may be sup- ( 

 posed would freely intercross, as varieties ; and those which 

 appear to have lost this power, as species. As the differ- 

 ences depend on the insects having long fed on distinct 

 plants, it cannot be expected that intermediate links con- 

 necting the several forms should now be found. The natur- 

 alist thus loses his best guide in determining whether to 

 rank doubtful forms as varieties or species. This likewise 

 necessarily occurs with closely allied organisms, which, in- 



