General Consider atio7is. xxi 



found living' is ordinarily correlated. The Ganoids among-st Fishes, 

 the Perennibranchiata among'st Amphibia, the Crinoids amongst 

 Echinodermata, and the Monotremata amongst Mammals, furnish 

 us with illustrations of these laws for the enunciation of which we 

 are indebted to Von Baer "'. 



A remark of the late Mr. W. S. Macleay, to the effect that no 

 character is natural until it has been proved to be so ^, has the merit 

 of at once expressing* tersely the necessity of constant recourse to 

 verification when we make deductions from general principles, and 

 of drawing attention to the striking morphological fact of the 

 varyhig value of class eharacters. In the face of statements as to 

 the eligibility of particular systems "^ as bases of classification, which 

 are only less sweeping and general than they are mutually con- 

 tradictory, it is a satisfaction to be able to quote the following 

 words from the writings of another English naturalist, the late 

 Professor Edward Forbes, — ' no character, whether of structure or 

 form, preserves an equal value in every tribe, but varies in its im- 

 portance, in one group characterizing a class, in another scarcely 

 determining a species ;' whilst the words of Macleay should be 



<= See Nova Acta, xiii., 2, p . 742, 1826, or Professor Huxley's Translation in 

 'Scientific Memoirs,' pp. 180, 181, 1853. 



^ See Linnaean Society's Transactions, xxiii., p. 75, i860. 



e For the applicability of the nervous system as a basis of classification, see Cuvier, 

 The Animal Kingdom, English Translation, 1854, p. 31 ; Lacaze Duthiers, Comptes 

 Rendus, 1865, torn, ii., p. 800; Blanchard, Ann. Sci. Nat., Ser. iii., tom. v., pp. 276, 

 376, 1846 ; Dana, Crustacea, pp. 46, 59 ; Waterhouse, Ann. and Mag. Nat. Hist., 

 vol. xii., p. 399, 1843 ; Professor Owen, Linnaean Society's Proceedings, 1857. For 

 the applicability of the Reproductive, see Dana, I.e., p. 62 ; Professor Owen, cit. 

 Darwin, Origin of Species, chap, xiii., p. 490, 4th ed. t866 ; Fischer, Orthoptera, 

 p. 62 ; Stein, Vergleichende Anatomic und Physiologic des Insekten ; erste Mono- 

 graphie ; Die weiblichen Geschlechts-Organe der Kafer, 1847, passim. For the appli- 

 cability of the Respiratory, see Dana, I.e., p. 62. For the value of the changes gone 

 through in development, see Professor Wyville Thomson, Phil. Trans., vol. 155, 

 pt. ii., pp. 514, 532, where attention is drawn to the power which circumstances of 

 light, warmth, aeration and nourishment have in modifying and hurrying over certain 

 stages of larval growth ; Oskar Schmidt, Sitzungsbericht, Nat. Wiss. Class. Kais. 

 Akad. Wien. xix., p. 193, 1856 ; Darwin, Animals and Plants under Domestication, 

 vol. ii., pp. 366-368 ; Origin of Species, p. 494, ihique citata. For the value of the 

 motor system as a basis for classification, in the sub-kingdom Echinodermata, see 

 Brandt, Prodromus, 1835. For that of the Placental system in the class Mammalia, 

 see Zool. Soc. Trans., vol. v., pt. 4, p. 285, 1865 ; H. Milne Edwards et Alphonse 

 Milne Edwards, Recherches pour servir k I'Histoire Naturelle des Mammifferes, 

 Livraison 1., p. 18, seqq., ihique citata, 1868. 



