Chap. XIII. CLASSIFICATION. 425 



But it may be asked, what ought we to do, if it could be 

 proved that one species of kangaroo had been produced, 

 by a long course of modification, from a bear ? Ought 

 we to rank this one species with bears, and what should 

 we do with the other species? The supposition is of 

 course preposterous ; and I might answer by the argu- 

 mentum ad hominem, and ask what should be done if a 

 perfect kangaroo were seen to come out of the womb of 

 a bear? According to all analogy, it would be ranked 

 with bears ; but then assuredly all the other species of 

 the kangaroo family would have to be classed under 

 the bear genus. The whole case is preposterous ; for 

 where there has been close descent in common, there 

 will certainly be close resemblance or affinity. 



As descent has universally been used in classing to- 

 gether the individuals of the same species, though the 

 males and females and larvae are sometimes extremely 

 different ; and as it has been used in classing varieties 

 which have undergone a certain, and sometimes a con- 

 siderable amount of modification, may not this same 

 element of descent have been unconsciously used in 

 grouping species under genera, and genera under higher 

 groups, though in these cases the modification has been 

 greater in degree, and has taken a longer time to com- 

 plete ? I believe it has thus been unconsciously used ; 

 and only thus can I understand the several rules and 

 guides which have been followed by our best system- 

 atists. We have no written pedigrees ; we have to 

 make out community of descent by resemblances of any 

 kind. Therefore we choose those characters which, as> 

 far as we can judge, are the least likely to have been 

 modified in relation to the conditions of life to which 

 each species has been recently exposed. Eudimentary 

 structures on this view are as good as, or even some- 

 times better than, other parts of the organisation. We 



