12 MR. A. R. WALLACE OX THE PAPILIOXID^ 



all seem equally constant ; and as most of these had abeady been named and described 

 as species, I have added the Xew Guinea form under the name of P. Penelojje. TTe thus 

 get a little group of Ulyssine PapiUos, the whole comprised within a very limited area, each 

 one confined to a separate portion of that area, and, though differing in various amounts, 

 each apparently constant. Few natui-alists wtU doubt that all these may and probably 

 have been derived from a common stock ; and therefore it seems desii'able that there 

 should be a unity in our method of treating them : either call them all varieties or all 

 species. Varieties, however, continually get overlooked ; in lists of species they are often 

 altogether unrecorded ; and thus Ave are in danger of neglecting the interesting phenomena 

 of variation and distribution which they present. I think it advisable, therefore, to name 

 all such forms ; and those who will not accept them as species may consider them as sub- 

 species or races. 



6. Species. — Species are merely those strongly marked races or local forms which, when 

 in contact, do not intermix, and when inhabiting distinct areas are generally believed to 

 have had a separate origin, and to be incapable of producing a fertile hybrid offspring. 

 But as the test of hybridity cannot be applied in one case in ten thousand, and even if it 

 could be applied, would prove nothing, since it is founded on an assumption of the very 

 question to be decided — and as the test of separate origin is in every case inapplicable — 

 and as, further, the test of non-intermixture is useless, except in those rare cases where 

 the most closely aUied species are found inhabiting the same area, it will be CArident 

 that we have no means whatever of distinguishing so-called " true species " from the 

 several modes of variation here pointed out, and into which they so often pass by an 

 insensible gi-adation. It is quite true that, in the great majority of cases, what we term 

 " species " are so well marked and definite that there is no difference of opinion about 

 them ; but as the test of a true theory is, that it accounts for, or at the veiy least is not 

 inconsistent with, the whole of the phenomena and apparent anomalies of the problem 

 to be solved, it is reasonable to ask that those who deny the origin of species by variation 

 and selection should grapple with the facts in detail, and show how the doctrine of the 

 distinct origin and permanence of species will explain and harmonize them. It has been 

 recently asserted by a high authority that the diflB.culty of limiting species is in propor- 

 tion to our ignorance, and that just as groups or countries are more accurately known 

 and studied in greater detail the limits of species become settled*. This statement has, 

 like many other general assertions, its portion of both truth and error. There is no doubt 

 that many uncertain species, founded on few or isolated specimens, have had theii- 

 true nature determined by the study of a good series of examples : they have been 

 thereby established as species or as varieties ; and the number of times this has occui*red 

 is doubtless very great. But there are other and equally trustworthy cases in which, not 

 single species, but whole groups have, by the study of a vast accumulation of materials, 

 been proved to have no definite specific limits. A few of these must be adduced. In 

 Dr. Carpenter's ' Introduction to the Study of the Foraminifera,' he states that " there is 

 not a single specimen of plant or animal of ichich the range of variation has been studied 

 by the collocation and comparison of so large a number of specimens as have passed under 

 the review of Messrs. Williamson, Farker, Rupert Jones, and myself, in our studies of the 

 * See Dr. J. E. Gray "On the Species of Lemuroids," Proc. Zool. Soc. 1863, p. 134. 



