INTRODUCTION. 79 



sudden transitions to new combinations, as in rumi- 

 nants, seals, and whales ; also solitary groups, like 

 the equine, and elephantine, and divergences in most 

 families, which tend, some towards one, others to- 

 wards another, and even to a third order, by means 

 whereof they osculate, more or less, in circles of 

 seeming affinities with surrounding families of other 

 forms, which, in their turn, have similar tendencies;* 

 hence a succession of orders, families, genera, and 

 species, in a written system, necessarily following 

 each other seriatim, some are constantly found in 

 the list, which would better approximate a group, 

 not the next in the writer's system ; and, as his 

 object will naturally be, to terminate the series of 

 mammalia, where they obviously assume characters 

 of other classes ; that is, of birds, reptiles, or fishes, 

 he is forced to conduct his links of successive con- 

 nection, so as to end with whales, who already have 

 the form of fishes ; or by some other arrangement of 

 affinities, lead his series towards the birds or reptiles. 

 While the opossums were confined to a few species, 

 * Such, for example, as the genus Paradoxurus, which is 

 plantigrade, and therefore assimilates in that and other re- 

 spects with Nasua, in the family of bears ; while, by the den- 

 tition, &c., it is retained in the digitigrade viverrine series, 

 which, in late classification, terminating with Crosarchus, pre- 

 cedes the Hyaenas. It was from the frequent recurrence of 

 these tendencies, that in England an attempt was made to 

 form the whole animal kingdom into quinary sets of orders, 

 each order again making five families, and these families 

 osculating similarly with one of the other, and forming a con- 

 tinual series, each returning into itself; and, finally, each of 

 the mammalian orders having one corresponding in the birds. 



