198 Mi-.W. S. MacLeay on certain general Laivs regulaiing 



with the uiuloubted fact, that /liatus or chasms are everywhere 

 in nature presenting themselves to the view. But this truth 

 by no means contradicts the Linnean maxim, that no saltus 

 exists in nature, akhough such has been esteemed its effect by 

 certain naturaUsts who have been in the habit of taking the 

 words hiatus and salttis as synonymous terms *. Thus the 

 series of the Stjstema Naturcc and of the Eegne Animal is not 

 natural where the Cetacea intervene between Quadrupeds and 

 Birds, but is perfectly consonant with nature where the Tor- 

 toises are made to follow these last. In the first case, there is 

 a saltus or leap from Quadrupeds to Birds over a group totally 

 dissimilai- to the latter ; there is, in short, an unnatural inter- 

 ruption of the la-w of continuity, which shocks not merely the 

 naturalist but the ordinary observer. In the other case there 

 is only an hiatus or chasm, which the discoveries of a future 

 day may fully occupy. Speaking therefore theoretically, it 

 may be affirmed that a saltus never did exist in nature ; and 

 it also may be argued, with great appearance of truth, that if 

 the hiatus are I'eal which so commonly occur in nature, they 

 did not always exist; or, in short, as M. Fries expresses himself, 

 *' Omnis sectio naturalis circulum per se clausum exhibct." 



Now this definition of a natural group could never have 

 been given by any person who was not aware of the distinction 

 to be made between affinity and analogy. But whenever two 

 parallel series of objects linked by affinity are drawn up in 

 array, the connexion of their extremes, that is, the formation 

 of the circle, becomes in that very moment, so far as I have 

 observed, more or less conspicuous. 



It follows, moreover, from admitting the existence of analo- 

 gical relations, or, in other words, from laying down the )iaral- 

 lelism of groups in different series of affinity, tiiat the number of 

 groups in these series must be the same. For were it other- 

 wise, — as for instance, supposing three groups to exist in one 

 complete series, and four in another, — it is clear that the paral- 

 lelism could not exist. But if this parallelism be real, which 

 has been, as shown above, asserted independently of each other 

 by several naturalists acting in different bi"anches of natural his- 

 tory, then the number of groups of the next lower order com- 

 posing a group of a given degree must be determinate. And 

 if, moreover, we accord to our author the accuracy of the fol- 

 lowing rule, namely, " Nunquam negligendum, unumquodque 



* It is to be regretted that Professor Diigald Stewart slioulil have been 

 led into this common error, and thus have acquired a somewhat erroneous 

 notion of the law of continuity as it refers to natural history. See the se- 

 cond part of his admirable Dissertation, :r. prefixed to vol. v. of the Supi'lc- 

 incnl to the Enct/clnpardia Brilannica, 



Ycmnim 



