ON THE STUDY OF ZOOLOGY 355 



amount of work which can be expressed in terms of the 

 ordinary forces of nature. The final object of physiology 

 is to deduce the facts of morphology, on the one hand, 

 and those of distribution on the other, from the laws of 

 the molecular forces of matter. 



Such is the scope of zoology. But if I were to content 

 myself with the enunciation of these dry definitions, I should 

 ill exemplify that method of teaching this branch of physical 

 science, which it is my chief business to-night to recommend. 

 Let us turn away then from abstract definitions. Let us 

 take some concrete living thing, some animal, the commoner 

 the better, and let us see how the application of common 

 sense and common logic to the obvious facts it presents, in- 

 evitably leads us into all these branches of zoological science. 



I have before me 3 lobster. When I examine it, what 

 appears to be the most striking character it presents ? Why, 

 I observe that this part which we call the tail of the lobster, is 

 made up of six distinct hard rings and a seventh terminal 

 piece. If I separate one of the middle rings, say the third, 

 I find it carries upon its under surface a pair of limbs or 

 appendages, each of which consists of a stalk and two 

 terminal pieces. So that I can represent a transverse 

 section of the ring and its appendages upon the diagram 

 board in this way. 



If I now take the fourth ring I find it has the same struc- 

 ture, and so have the fifth and the second ; so that, in each 

 of these divisions of the tail, I find parts which correspond 

 with one another, a ring and two appendages ; and in each 

 appendage a stalk and two end pieces. These corresponding 

 parts are called, in the technical language of anatomy, 

 " homologous parts." The ring of the third division is the 

 " homologue " of the ring of the fifth, the appendage of the 

 former is the homologue of the appendage of the latter. And, 

 as each division exhibits corresponding parts in corresponding 

 places, we say that all the divisions are constructed upon the 

 same plan. But now let us consider the sixth division. It 

 is- similar to, and yet different from, the others. The ring 

 is essentially the same as in the other divisions ; but the 

 appendages look at first as if they were very different ; 

 and yet when we regard them closely, what do we find ? 

 A stalk and two terminal divisions, exactly as in the others, 

 but the stalk is very short and very thick, the terminal 

 divisions are very broad and flat, and one of them is divided 

 Into two pieces. 



