24 THE PRESENT CONDITION 



creatures wbicli have backbones, ribs, and legs, and 

 other parts arranged in the same general manner, and 

 in all their formation exhibiting the same broad pecu- 

 liarities. 



I am sure that you cannot have followed me even 

 in this extremely elementary exposition of the struc- 

 tural relations of animals, without seeing what I have 

 been driving at all through, which is, to show you that, 

 step by step, naturalists have come to the idea of a 

 unity of plan, or conformity of construction, among 

 animals which appeared at first sight to be extremely 

 dissimilar. 



And here you have evidence of such a unity of plan 

 among all the animals which have backbones, and 

 which we technically call Vertebrata. But there are 

 multitudes of other animals, such as crabs, lobsters, 

 spiders, and so on, which we term Annulosa. In these 

 I could not point out to you the parts that correspond 

 ■with those of the Horse, — the backbone, for instance, — 

 as they are constructed upon a very different prin- 

 ciple, which is also common to all of them ; that is to 

 say, the Lobster, the Spider, and the Centipede, have 

 a common plan running through their whole arrange- 

 ment, in just the same way that the Horse, the Dog, 

 and the Porpoise a.ssimilate to each other. 



Yet other creatures — whelks, cuttlefishes, oysters, 

 snails, and all their tribe [Mollusca) — resemble one 

 another in the same way, but differ from both Verte- 

 brata and Annulosa ; and the like is true of the animals 

 called Cceknterata (Polypes) and Protozoa (animalcules 

 and sponges). 



Now, by pursuing this sort of comparison, naturalists 



