INTKODUCTION. xxxi 



to all known rules, ** When Zoology," says Mr, Milne Edwards, " is only 

 studied in systematic works, it is often supposed that each class, each 

 family, and each genus, present to us boundaries precisely defined, and 

 that there can be no uncertainty as to the place to be assigned, in a natural 

 classification, to every animal the organisation of which is sufficiently 

 known. But when we study this science from Nature herself, we are soon 

 convinced of the contrary, and we sometimes see the transition from one 

 plan of structure to an entirely different scheme of organisation take place 

 by degrees so completely shaded one into the other that it becomes very 

 difficult to trace the line of demarcation between the groups thus con- 

 nected." — Ann. Sc. Nat. 1840, Sept. Ray long ago pointed this out in a 

 very remarkable passage, which cannot be too often quoted. 



" Verum quod alias dixi illud hie repeto et inculco, non sperandam k me 

 Methodum undequaque perfectam et omnibus suis numeris absolutam, quae 

 et plantas in genera ita distribuat ut imiversse species comprehendantur, 

 nulla adhuc anomala et sui generis reliqu^, et unumquodque genus notis 

 Buis propriis et characteristicis ita circumscribat, ut nuUse inveniantur 

 species incerti, ut ita dicam, laris, et ad plura genera revocabiles. Nee 

 enim id patitur natura rei. Nam, cum Natura (ut dici solet) non faciat 

 saltus, neque ab extreme ad extremum transeat nisi per medium, inter 

 superiores et inferiores, rerum ordines nonnullas mediae et ambiguse condi- 

 tionis producere solet, quse de utroque participent, et utrosque velut con- 

 nectant, ut ad utrum pertineant omnino incertum sit, Prseterea eadem 

 alma parens in methodi cujuscunque angustias cocrceri repugnat, sed ad 

 libertatem et avrovoiiiav suam nullis legibus obnoxiam ostentandam, in 

 unoquoque rerum ordine nonnullas species creare solet, tanquam exceptiones 

 a regulis generalibus, singulares et anomalas." — (Raii, Hist. Plant, vol. i. 

 Prsef.) Linneeus did but copy this when he asserted that Nature makes no 

 leaps [Natura non facit saltus. — Phil. Bot. 77.) 



This doctrine has, however, been lately called in question by no less 

 eminent a writer than M. Alphonse De CandoUe, who requires that absolute 

 limits should be assigned to all groups of whatever degree. "If," he says, 

 *'we cannot state in what respect two families differ permanently and 

 universally, those two families are but one. Two pieces of land which 

 touch each other form one island, and not two ; but two pieces of land 

 which are separated by an arm of the sea, form two islands, and not one." 

 — Annales des Sciences, series 3, vol. 1. p. 254. But this is a kind of 

 reasoning wholly inapplicable to Natural History, for the reasons so ad- 

 mirably given by Ray, and is contrary to all experience. If the groups 

 limited by M. Alphonse De Candolle himself are examined by this standard 

 they alone suffice to demonstrate how visionary are such expectations. Mr. 

 Bentham has satisfactorily answered the learned Botanist of Geneva. 

 " We Botanists," he says, ** cannot be so mathematically exact as geogra- 

 phers, and where an isthmus is very narrow, we must class the peninsula 

 with the island. How often does it happen that two large Orders, say of 

 five hundred to two thousand or three thousand species, totally distinct 

 from each other in all those species by a series of constant characters, are 

 yet connected by some small isolated genus of a dozen, half a dozen, nay a 

 single species, in which these very characters are so inconstant, uncer- 

 tain, or variously combined as to leave no room for the strait through 

 which we ought to navigate between the two islands." — London Journal 

 of Botany, 4. 232. It would be very convenient to find that the views of 

 M. Alphonse De Candolle were practicable, but in truth they are quite 

 Utopian. 



