68 Resemblances and Di^erences Among Living Things 



one type was created before rather than after a certain other 

 type. Some kind of fish, for example, or fishes in general, 

 might have been created either before birds or after birds. 

 Sea stars could have been created after lobsters as well as 

 before lobsters. There is nevertheless a remarkable coinci- 

 dence between the actual succession of fossils representing 

 successive periods of time and the succession of forms which 

 the idea of evolution would lead us to expect. New species 

 or forms appeared after similar but relatively simpler ones 

 already existed. 



The Test of a Species 



One of the oldest ideas about the permanence of species 

 is found in the assumption that the members of each species 

 will breed among each other, but not with members of another 

 species. This is indeed the basis for the notion of '' varieties," 

 as distinguishable subdivisions of a species, which are yet un- 

 related to other species. When the various breeds of dogs 

 or horses are crossed, we decide that they represent varieties, 

 not species. When the horse is crossed with the ass, we con- 

 tinue to call the horse and the ass separate species, and fall 

 back upon the sterility of the mule as evidence that the two 

 species are really not fertile although thousands of mules 

 carry on their dull but useful existence. 



There are other cases of crossing that are even more 

 striking, however. Some of these inter-species crosses are 

 those between the finch and the canary; between domestic 

 cattle and the bison; between the wolf and domestic dogs; 

 between the camel and the dromedary; between the zebu 

 and the yak; between the sika deer and the red deer; between 

 the goose and the pheasant. Recently there has been reported 

 a successful cross between the radish and the cabbage. Dif- 

 ferent strains of what is unquestionably the same " species " 

 sometimes prove to be sterile when mated. This does not 

 destroy the usefulness of the notion of species. On the other 

 hand, we are not warranted by the facts of such crossings 



