50 DESULTORY SKETCHES IN NATURAL HISTORY. 



It is difficult to avoid digressing when treating on systematic na- 

 tural history, as every statement requires its proof, which may in- 

 volve the discussion of miscellaneous topics. We might next pro- 

 ceed to notice the excessive irregularity of groups of equal value, 

 as shown by every conceivable mode of variation ; viz. with respect 

 to the number of species they may severally comprise, or may have ' 

 comprised during former epochs of the earth's history ; the number 

 of separate minor types they may comprehend, which bears no pro- 

 portion to that of species ; also the amount of adaptive modification 

 they may respectively exhibit, which again is equally indefinite, 

 both as regards the number of species and sub-typical forms ; and, 

 lastly, the distribution of them in the horizontal as well as vertical 

 series, which can as little be reduced to rule or proposition, some 

 extensively represented types being strictly confined to particular 

 regions or periods of time, while others are in either respect, or both, 

 of general difiusion, or they are circumscribed at the present era, 

 though formerly spread over a wide area, &c.* But, reserving 

 these various subjects for future comment in the pages of The Ana^ 

 lystf we will close the present introductory remarks, into which we 

 have almost unconsciously been led, by exemplifying the analogous 

 modification of diverse types, or the relation of what is currently 

 termed analogy, as distinguished from affinity. A remarkable illus- 

 tration of this superficial similitude, induced by the correspondency 

 of the adaptive modifications in reference to habit, such as occasions 

 the Cetacea to assume the outside form of fishes, while they retain 

 every essential characteristic of their class (Mammalia), is afforded 

 in the class of birds, by the familiarly-known genera of the Swifts 

 and the Swallows, which almost every systematist (and, we believe, 

 without exception, every British systematist) has erred in placing 

 together in zoological classification. The two genera in question 

 are alike modified for seeking and capturing their insect prey on the 

 wing ; and are both furnished, therefore, with a remarkably wide 

 gape, long wings, and generally a forked tail to assist in steering ; 

 while their legs (which are little used for progression) are short 

 and inconspicuous. But here their whole similitude ceases; for 

 they differ astonishingly in every detail of their conformation, which 



necessary to include him in a particular group with the Apes, whatever may 

 be the notions entertained of his distinctness in other respects, with which 

 zoology has no sort of concern. 



* Strange that, in the face of such conspicuously obvious facts, numerous 

 zoologists should still contend for uniformity in the amount of variation of 

 groups, as implied by their ternary, quinary, septennary systems, &c. ! 



