vn THE STUDY OF ZOOLOGY 205 



Gradually investing the yolk, it became subdivided 

 by transverse constrictions into segments, the 

 forerunners of the rings of the body. Upon the 

 ventral surface of each of the rings thus sketched 

 out, a pair of bud-like prominences made their ap- 

 pearance 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 

 doctrine 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 applica- 

 tion. 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 



