34 DOUBTFUL SPECIES. [CHAP. II. 



forms have permanently retained their characters for a long time ; 

 for as long, as far as we know, as have good and true species. 

 Practically, when a naturalist can unite by means of intermediate 

 links any two forms, he treats the one as a variety of the other ; 

 ranking the most common, but sometimes the one first described, 

 as the species, and the other as the variety. But cases of great 

 difficulty, which I will not here enumerate, sometimes arise in 

 deciding whether or not to rank one form as a variety of another, 

 even when they are closely connected by intermediate links ; nor 

 will the commonly-assumed hybrid nature of the intermediate 

 forms always remove the difficulty. In very many cases, however, 

 one form is ranked as a variety of another, not because the inter- 

 mediate links have actually been found, but because analogy leads 

 the observer to suppose either that they do now somewhere exist, 

 or may formerly have existed ; and here a wide door for the entry 

 of doubt and conjecture is opened. 



Hence, in determining whether a form should be ranked as a 

 species or a variety, the opinion of naturalists having sound judg- 

 ment and wide experience seems the only guide to follow. We 

 must, however, in many cases, decide by a majority of naturalists, 

 for few well-marked and well-known varieties can be named which 

 have not been ranked as species by at least some competent 

 judges. 



That varieties of this doubtful nature are far from uncommon 

 cannot be disputed. Compare the several floras of Great Britain, 

 of France, or of the United States, drawn up by different botanists, 

 and see what a surprising number of forms have been ranked by 

 one botanist as good species, and by another as mere varieties. 

 Mr. H. C. Watson, to whom I lie under deep obligation for assist- 

 ance of all kinds, has marked for me 182 British plants, which 

 are generally considered as varieties, but which have all been 

 ranked by botanists as species ; and in making this list he has 

 omitted many trifling varieties, but which nevertheless have been 

 ranked by some botanists as species, and he has entirely omitted 

 several highly polymorphic genera. Under genera, including the 

 most polymorphic forms, Mr. Babington gives 251 species, whereas 

 Mr. Bentham gives only 112, a difference of 139 doubtful forms ! 

 Amongst animals which unite for each birth, and which are highly 

 locomotive, doubtful forms, ranked by one zoologist as a species 

 and by another as a variety, can rarely be found within the same 

 country, but are common in separated areas. How many of the 

 birds and insects in North America and Europe, which differ very 

 slightly from each other., have been ranked by one eminent 

 naturalist as undoubted species, and by another as varieties, or, 

 as they are often called, geographical races ! Mr. Wallace, in 

 several valuable papers on the various animals, especially on the 



