Stitbg 0f &0oI0gjr. ioi 



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 ; atnd 

 it was only at a later period, that by ,the* modification; o'r 

 absorption, of certain of these primitive "consfaiueiitS) 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 w^ay 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 

 ddult 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 

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

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

 ike the lobster. But other animals, though they may 

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

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



