VARIATION UNDER NATURE 39 



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 him- 

 self, 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 de- 

 pend 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 

 determining 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 in- 

 habits many islands in the same archipelago, and presents different 

 forms in the different areas, there 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 vari- 

 eties. 



Some few naturalists maintain that animals never present vari- 

 eties ; 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 be- 

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

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

 implying 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, hy- 

 bridism, etc., have been brought to bear in the attempt to deter- 

 mine their rank; but space does not here permit me to discuss 

 them. Close investigation, in many cases, will no doubt bring natu- 

 ralists to agree how to rank doubtful forms. Yet it must be con- 

 fessed that it is in the best known countries that we find the great- 

 est 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 closely attracts his attention, varieties of it will 



