vi.] ON THE STUDY OF ZOOLOGY. 87 



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 sub 

 divided 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, ; n 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 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, bat, 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 



