176 Phenomena of Variation and Geographical Distribution. 



the most closely allied species are found inhabiting the same area, it 

 will be evident 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 gradation. 

 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 ac- 

 counts for, or at the very least if 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 varia- 

 tion 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 difficulty of limiting species is in proportion to our 

 ignorance, and that just as groups or countries are more accurately 

 known and studied in greater detail the hmits of sjDecies become set- 

 tled. This statement has, like many other general assertions, its por- 

 tion of both truth and error. There is no doubt that many uncertain 

 species, founded on few or isolated specimens, have had their true na- 

 ture 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 occurred is doubtless very great. But there are other 

 and equally trustworthy cases in which, not single species, but whole 

 gi'oups have, by the study of a vast accumulation of materials, been 

 proved to have no definite specific limits. A few of these must be ad- 

 duced. In Dr. Carpenter's "Introduction to the Study of the Foram- 

 inifera," he states that "there is not a single specimen of plant or ani- 

 mal of which the range of variation has been studied by the colloca- 

 tion and comparison of so large a number of specimens as have passed 

 under the review of Messrs. Williamson, Parker, Rupert Jones, and 

 myself, in our studies of the types of this group ; " and the result of 

 this extended comparison of specimens is stated to be, " The range of 

 variation is so great among the Foraminifera as to include not merely 

 those difl^erential characters which have been usually accounted speci- 

 fic, but also those upon which the greater part of the genera of this 

 group have been founded, and even in some instances those of its or- 

 ders" (Foraminifera, Preface, x.) Yet this same group has been 

 divided by D'Orbigny and other authors into a number of clearly de- 

 fined /amines, genera, and speciea, which these careful and conscientious 

 researches have shown to have been almost all founded on incomplete 

 knowledge. 



Professor DeCandolle has recently given the results of an extensive 



