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Show 24 THE PRESENT CONDITION creatures which 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 structural 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 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 IIorse, 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 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 OF ORGANIC NATURE. 25 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 tern pted to go on a step and inquire 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 existence with one and the sa1ne primitive form,-that of the egg, consisting, as we have seen, of a jntrogenous 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 kingdom which has been guessed at and fancied for many years; but which it has been left to the present time to be demonstrated by the careful study of development. But is it possible to go another step further still, and to show that in the same way the whole of the organic world is reducible to one primitive condition of form? Is there among the plants the same primitive form of organization, and is that identical with that of the animal kingdom ? The reply to that |