326 THE CAUSES OF THE xi 



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 principle, 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 arrangement, 

 in just the same way that the horse, the dog, 

 and the porpoise assimilate to each other. 



Yet other creatures whelks, cuttlefishes, 

 oysters, snails, and all their tribe (Molluscd) 

 resemble one another in the same way, but differ 

 from both Vertebrata and Annulosa ; and the like 

 is true of the animals called Ccelenterata (Polypes) 

 and Protozoa (animalcules and sponges). 



Now, by pursuing this sort of comparison, 

 naturalists have arrived at the conviction that 

 there are, some think five, and some seven, but 

 certainly not more than the latter number and 

 perhaps it is simpler to assume five distinct plans 

 or constructions in the whole of the animal world ; 

 and that the hundreds of thousands of species 

 of creatures on the surface of the earth, are all 

 reducible to those five, or, at most, seven, plans of 

 organisation. 



But can we go no further than that ? When 

 one has got so far, one is tempted to go on a step 



