22 THE ANATOMY OF INVERTEBRATED ANIMALS. 



canal and its derivatives ; and the mesoblast, to intermediate 

 structures. The tendency of recent inquiry is to prove that 

 the several layers of the germ evolve analogous organs in in- 

 vertebrate animals, and to indicate the possibility of tracing 

 the several germ-layers back to the blastomeres of the yelk, 

 from the subdivision of which they proceed. 



It is conceivable that all the forms of life should have pre- 

 sented about the same differentiation of structure, and should 

 have differed from one another by superficial characters, each 

 form passing by insensible gradations into those most like it. 

 In this case Taxonomy, or the classification of morphological 

 facts, would have had to confine itself to the formation of a 

 serial arrangement, representing the serial gradation of these 

 forms in Nature. 



It is conceivable, again, that living beings should have dif- 

 fered as widely in structure as they actually do, but that the 

 interval between any two extreme forms should have been 

 filled up by an unbroken series of gradations ; in which case, 

 again, classification could only affect the formation of series 

 the strict definition of groups would be as impossible as in the 

 former case. 



As a matter of fact, living beings differ enormously, not 

 only in differentiation of structure, but in the modes in which 

 that differentiation is brought about ; and the intervals be- 

 tween extreme forms are not filled up, in the existing world, 

 by complete series of gradations. Hence it arises that living 

 beings are, to a great extent, susceptible of classification into 

 groups, the members of each group resembling one another, 

 and differing from all the rest, by certain definite peculiarities. 



No two living- beings are exactly alike, but it is a matter 

 of observation that, among the endless diversities of living 

 things, some constantly resemble one another so closely that 

 it is impossible to draw any line of demarkation between them, 

 while they differ only in such characters as are associated 

 with sex. Such as thus closely resemble one another consti- 

 tute a morphological species while different morphological 

 species are defined by constant characters which are not 

 merely sexual. 



The comparison of these lowest groups, or morphological 

 species, with one another, shows that more or fewer of them 

 possess some character or characters in common some feat- 

 ure in which they resemble one another and differ from all 

 other species and the group or higher order thus formed is 



