26 THE PKESENT CONDITION 



common plan running through their whole arrange- 

 ment, 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 (Mollusca) — resemble one 

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

 brata and Annulosa ; and the like is true of the ani- 

 mals called Coelenterata (Polypes) and Protozoa (ani- 

 malcules 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 organization. 



But can we go no further than that ? When one 

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

 quire whether we cannot go back yet further and bring 

 down the whole to modifications of one primordial 

 unit. The anatomist cannot do this ; but if he call to 

 his aid the study of development, he can do it. For 

 we shall find that, distinct as those plans are, whether 

 it be a porpoise or man, or lobster, or any of those 

 other kinds I have mentioned, every one begins its ex- 

 istence with one and the same primitive form, — that of 

 the egg, consisting, as we have seen, of a introgenous 

 substance, having a small particle or nucleus in the 

 centre of it. Furthermore, the earlier changes of each 

 are substantially the same. And it is in this that lies 

 that true " unity of organization " of the animal king- 



