1038 The Zoologist — January, 1868. 



reptiles and fishes, as four equivalent classes of endosteate animals, but 

 subsequent writers have divided the reptiles into two classes, first to 

 gratify a fiction, once fashionable, that Nature prefers the number 

 five, and, secondly and more remotely, because there was an intrinsic 

 difference in their mode of reproduction : this difference is a true one, 

 but I have shown elsewhere that it obtains also in sticklers, birds and 

 fishes.* The difference is most strongly pronounced in fishes: it has 

 its second degree of development in reptiles; its third in birds; and 

 its fourth in sucklers. It is very interesting to observe how beautifully 

 similarities develope themselves in the component parts of these dicho- 

 tomous divisions as soon as this principle is understood : it is only 

 necessary to glance at a lizard and a salamander, both comprised by 

 Linneus in the genus Lacerta, but totally different in physiological 

 characters, to appreciate and understand the whole theory of pairs- 

 Two beings enter on life side by side, totally different from each other, 

 and having a different external form, a different system of circulation 

 and respiration, eating different food, the one aquatic, the other 

 terrestrial, yet approaching each other hour by hour, week by week, 

 until they become, in the estimation of a Linneus, members of the same 



genus, Lacerta. 



It is exactly thus amongst insects; they commence life differently 

 and end it alike, so truly alike that it is the pleasure, I may say the 

 vocation, of some entomologists, to insist they are identical ; and in 

 those few instances in which this assertion is not made, as in the case 

 of Psi and trideus, it is because these have, from long custom, acquired 

 a prescriptive right to specific rank, not because any specific diagnosis 

 has ever been proposed. 



These natural dichotomies contrast in the most striking manner with 

 scientific dichotomies, of which there is none more popular than those 

 consisting of a positive and a negative ; even such dichotomies are 

 truthful but vague, like the poet's division of the world into two 



moieties : 



" The one that small 

 Beloved and consecrated spot 

 Where Leah was, the other all 



The dull wide waste where she was not." 



One moiety will be acknowledged to be clear and definite, the other 

 vague, indefinite, and somewhat too comprehensive for our just 

 apprehension. 



* ' Essay on the Physiological Classification of Auiinals.' Van Voorst, 1852. 



