358 ON THE STUDY OF ZOOLOGY 



trace of any one of those organs, whose multiplicity and 

 complexity, in the adult, are so surprising. After a time 

 a delicate patch of cellular membrane appeared upon one 

 face of this yolk, and that patch was the foundation of the 

 whole creature, the clay out of which it would be moulded. 

 Gradually investing the yolk, it became subdivided by 

 transverse constrictions into segments, the fore-runners of 

 the rings of the body. Upon the ventral surface of each 

 of the rings thus sketched out, a pair of bud-like promi- 

 nences made 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 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, indis- 

 tinguishable. 



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 every- 

 where 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 fish, the rock lobster, and the prawn, and the 



