1864.] DR. DAWSON ON CLASSIFICATION OF ANIMALS. 247 



the genus Corvus or in the fiimily Ooj-vidce, we express merely our 

 belief in a certain structural resemblance, not in any genetic con- 

 nection. Nor need we suppose that if any of the species of a genus 

 were destroyed they would be reproduced from the others. Further, 

 while all the individuals of any of the species may be precisely 

 similar to each other and still be distinct individuals, all the species 

 of the genus cannot be similar in all their characters, otherwise 

 they would constitute but one species. 



In other words, the species and the genus, considered as groups, 

 differ not in degree but in kind. To make this very plain, let us 

 take a familiar illustration. I have a number of maps, all uniform 

 in size and in style of execution ; but in the whole there are only two 

 kinds, — maps of the eastern hemisphere, and maps of the western 

 hemisphere. Now all of the maps of one kind constitute a species ; 

 those of both kinds, a genus. The individuals of one species, say 

 of the eastern hemisphere, are all alike. They have all been struck 

 from one plate, from which many similar maps may be produced. 

 But the other map, though necessary to make up the set or genus, 

 may be quite dissimilar in all its details from the first, and could 

 not be produced from its plate. We have no difl&culty here in 

 understanding that the specific unity is of a different kind from the 

 generic unity, and that the distinction is by no means one of mere 

 grade of resemblance. Avery little thought must convince any one 

 that this applies to species and genera in zoology ; and that those 

 naturalists who affirm that species have no more real existence in 

 nature than genera, have overlooked one of the essential elements 

 of classification. Nor would this distinction be invalidated by 

 the assumption of a descent with modification, unless it could be 

 shown that in actual nature species shade into each other ; and 

 this is certainly not the case in those which are reckoned as good 

 species. 



I have been thus careful to insist on the nature of the species 

 in natural history ; because I believe that loose views on this 

 subject have caused a large proportion of the errors in classifica- 

 tion. 



Though the groups higher than species do not exist in nature 

 in the same sense in which species exist, they are not arbitrary, 

 but depend on our conception of resemblances and differences which 

 actually exist. We go out into the forest and perceive different 

 species of trees ; but, at the same time, we find that these species 

 can be grouped in genera, as Oaks, Birches, Maples, &c., under 



