vi.) ON THE STUDY OF ZOOLOGY. 101 



their appearance — the rudiments of the appendages of 

 the ring. At first, all the appendages were alike, but, as 

 they grew, most of them became distinguished, into a 

 stem and. two terminal divisions, to which, in the middle 

 part of the body, was added a third outer division ; and 

 it was only at a later period, that by the modification, or 

 absorption, of certain of these primitive constituents, the 

 limbs acquired, their perfect form. 



Thus the study of development proves that the doc- 

 trine of unity of plan is not merely a fancy, that it is 

 not merely one way of looking at the matter, but that it 

 is the expression of deep-seated natural facts. The legs 

 and jaws of the lobster may not merely be regarded as 

 modifications of a common type, — in fact and in nature 

 they are so, — the leg and the jaw of the young animal 

 being, at first, indistinguishable. 



These are wonderful truths, the more so because the 



zoologist finds them to be of universal application. The 



investigation of a polype, of a snail, of a fish, of a horse, 



or of a man, would have led us, though by a less easy 



path, perhaps, to exactly the same point. Unity of plan 



everywhere lies hidden under the mask of diversity of 



structure — the complex is everywhere evolved out of the 



simple. Every animal has at first the form of an egg, 



and every animal and every organic part, in reaching its 



adult state, passes through conditions common to other 



animals and other adult parts ; and this leads me to 



another point. I have hitherto spoken as if the lobster 



were alone in the world, but, as I need hardly remind 



you, there are myriads of other animal organisms. Of 



these, some, such as men, horses, birds, fishes, snails, 



slugs, oysters, corals, and sponges, are not in the least 



like the lobster. But other animals, though they may 



differ a good deal from the lobster, are yet either very 



like it, or are like something that is like it. The cray 



