xxii Introduction. 



borne in mind with reference to certain other statements ^ of even 

 greater generality as to the ajiplicability or inapplicability of phy- 

 siological differences as bases of zoological arrangements. Nothing 

 is easier than to say that by the nervous, or by the reproductive, 

 or by the respiratory systems, or by the history of the changes 

 gone through in development, ' characters of the widest bearing in 

 classification are furnished ;' but nothing is more certain, as a 

 verification of statements referred to below will demonstrate, than 

 that what is true of one of these bases of classification within the 

 limits of one Sub-kingdom, or within the limits of one Class, or even 

 within the limits of yet smaller groups, will not be by any means 

 invariably found to be true within the limits of another similar 

 division. Our knowledge, again, of the power which organisms 

 have of adjusting themselves to their environment, may inchne us 

 to think the motor and tegumentary systems to be bad bases for 

 classification, as it is through them that the animal comes mainly 

 and mostly into relation with external influences. Yet, if Seals 

 and Whales exhibit marks of their affinities to the Carnivora and 

 the Artiodactyla respectively, even in such matters as the cha- 

 racter of their placentae, the number of the bronchi in their lungs, 

 and, in spite of the modifications which their motor and tegu- 

 mentary systems have undergone, it is nevertheless true that the 

 specialization of the same systems in Aves and Echinodermata 

 appears to have entailed corresponding variations throughout the 

 entirety of theu* respective organisms, and in organs of vegetable 

 as well as in organs of animal life. 



The facts of the varying morphological value of zoological dif- 

 ferentiae ; of the unequal quantitative extent of divisions of equal 

 morphological rank ; and of the unequal distances separating such 

 divisions, go some way towards accounting for the arbitrary way 

 in which the same division has had very different morphological 

 rank assigned to it by different classificatory writers. A pro- 

 visional character however must always attach itself to a greater 

 or smaller part of all our classifications ; if they succeed in pre- 



' See Erichson, Entomographien, p. i, 1840, 'Dies ist ein physiologischer, kein 

 ' zoologischer Character/ Semper, Reisen in Archipel der Philippinen, Theil. ii., Bd. i., 

 p. 52; Carpenter, Foraminifera, Ray Society, p. 14, 1863 ; Herbert Spencer, Prin- 

 ciples of Biology, vol. i., pp. 306, 307, 1864 ; Darwin, Origin of Species, 4th ed., 

 p. 490. 



