378 PRINCIPLES AND METHODS OF PALAEONTOLOGY. 



Seas ever communicated, the Nephrops would readily have spread from 

 Norway to the Adriatic, or vice versa ; and when the central mass of 

 Europe rose again, the area of its distribution would be cut in two, and 

 the northern and southern fragments only left. 



That this is the explanation of the apparent anomaly would be proved 

 if Neplirops norvegicus were found fossil in any of the strata constituting 

 the present land of Central Europe. So long as this is not the case it 

 can only be regarded as hypothesis more probable than that of special 

 creation at two points, and hence excluding the necessity of adopting 

 the latter.* 



Many cases of distribution which have been supposed to be similar 

 to that of Nephrops, and adduced as such by the advocates of many 

 centers of creation, have been shown to be not really of the same nature, 

 the widelj' separated forms not being in reality of identical species. 



c. The great question, however, upon which the two schools of natu- 

 ralists divide is: Are species permanent? In other words, is it possible 

 that any conditions operating through any amount of time upon any 

 number of generations of a species A, shall give rise to a distinct spe- 

 cies B ? 



In this, as in all other instances where thinking men entertain flatly 

 contradictory opinions, the difficulty of coming to a mutual understand- 

 ing appears to arise in a great measure from the want of a clear appre- 

 hension of one another's meaning. In the present case it is probable 

 that no two persons attach precisely the same signification to th6 word 

 ^' species." 



Most naturalists admit, indeed, {hat species have a distinct physiolo- 

 gical character, viz: that the intermixture of two species will not pro- 

 duce a fertile race, even if it gives rise to any progeny at all ; but, un- 

 fortunately, this test is, from the nature of the case, practically inappli- 

 cable, not only to the great majority of living animals and plants, but 

 to all fossils. 



In practice, therefore, the naturalist is obliged to neglect the physio- 

 logical characters of a species, and to confine himself entirel}^ to those 

 which can be founded on form and structure. In this sense a species 

 is the smallest group to which distinctive and invariable characters can 

 be assigned. 



If, to use a seemingly paradoxical expression, all living beings were 

 extinct — if they were represented by a limited number of fossils, and 

 lay before us as things to be arranged and classified, the practical aj^pli- 

 cation of this definition of species would have no difficulty. Sooner or 

 later the whole organic world would be sorted out into the smallest 

 parcels which could be characterized by a definition, and these would 

 be " species." 



It is obvious that the task would be equiUly easy were all living 

 beings absolutely immutable ; if every member of a species were exactly 

 like its fellows, and if all progeny precisely resembled its parentage. 



If every dog, for example, were precisely like every other dog, and 

 every puppy exactly similar to its parents, there could be no difficulty 

 about defining the species dog, nor could there be any hesitation iu 

 deciding whether a given animal belonged to the species dog or the 

 species wolf. 



Unfortunately for scientific ease, no such immutable forms exist in 

 nature. Like everything else in the world, a living being is a compro- 



* Species of the genus Neplirops have, curiously enousb, been found fossil in Central 

 Franco (deiiartment of the Youue) at a point about half way between the northern and 

 Bouthern area of N. norvegicus. 



