Chap. II.] DOUBTFUL SPECIES. 30 



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 differences are ratber more 

 strongly marked, and when both sexes and all ages are affected, the 

 forms are ranked by all entomologists 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. Walsh ranks the forms which it may be 

 supposed would freely intercross, as varieties; and those which 

 appear to have lost this power, as species. As the differences depend 

 on the insects having long fed on distinct plants, it cannot be 

 expected that intermediate links connecting the several forms should 

 now be found. The naturalist thus loses his best guide in deter- 

 mining whether to rank doubtful forms as varieties or species. This 

 likewise necessarily occurs with closely allied organisms, which 

 inhabit distinct continents or islands. When, on the other hand, 

 an animal or plant ranges over the same continent, or inhabits many 

 islands in the same archipelago, and presents different forms in the 

 different areas, thera is always a good chance that intermediate 

 forms will be discovered which will link together the extreme 

 states ; and these are then degraded to the rank of varieties. 



Some few naturalists maintain that animals never present varie- 

 ties ; but then these same naturalists rank the slightest difference 

 as of specific value; and when the same identical form is met with 

 in two distant countries, or in two geological formations, they 

 believe that two distinct species are hidden under the same dress. 

 The term species thus comes to be a mere useless abstraction, im- 

 plying and assuming a separate act of creation. It is certain that 

 many forms, considered by highly-competent judges to be varieties, 

 resemble species so completely in character, that they have been thus 

 ranked by other highly-competent judges. But to discuss whether 

 they ought to be called species or varieties, before any definition of 

 these terms has been generally accepted, is vainly to beat the air. 



Many of the cases of strongly-marked varieties or doubtful species 

 well deserve consideration ; for several interesting lines of argument, 

 from geographical distribution, analogical variation, hybridism, &c, 

 have been brought to bear in the attempt to determine their rank ; 

 but space does not here permit me to discuss them. Close investi- 

 gation, in many cases, will no doubt bring naturalists to agree how 

 to rank doubtful forms. Yet it must be confessed that it is in the 

 best known countries that we find the greatest number of them. 

 I have been struck with the fact, that if any animal or plant in a 

 state of nature be highly useful to man, or from any cause closelv 



