86 LAY SERMONS, ESSAYS, AND REVIEWS. [vi. 



in the foremost jaw, the so-called mandible, the inner division only 

 is left ; and, in the same way, the parts of the feelers and of the 

 eye-stalks can be identified with those of the legs and jaws. 



But whither does all this tend ? To the very remarkable 

 conclusion that a unity of plan, of the same kind as that 

 discoverable in the tail or abdomen of the lobster, pervades the 

 whole organization of its skeleton, so that I can return to the 

 diagram representing any one of the rings of the tail, which I 

 drew upon the board, and by adding a third division to each ap 

 pendage, I can use it as a sort of scheme or plan of any ring of 

 the body. I can give names to all the parts of that figure, and 

 then if I take any segment of the body of the lobster, I can 

 point out to you exactly, what modification the general plan has 

 undergone in that particular segment ; what part has remained 

 movable, and what has become fixed to another; what has 

 been excessively developed and metamorphosed and what has 

 been suppressed. 



But I imagine I hear the question, How is all this to be 

 tested ? No doubt it is a pretty and ingenious way of looking 

 at the structure of any animal, but is it anything more ? Does 

 Nature acknowledge, in any deeper way, this unity of plan we 

 seem to trace ? 



The objection suggested by these questions is a very valid and 

 important one, and morphology was in an unsound state so long 

 as it rested upon the mere perception of the analogies which 

 obtain between fully formed parts. The unchecked ingenuity of 

 speculative anatomists proved itself fully competent to spin any 

 number of contradictory hypotheses out of the same facts, 

 and endless morphological dreams threatened to supplant 

 scientific theory. 



Happily, however, there is a criterion of morphological truth, 

 and a sure test of all homologies. Our lobster has not always 

 been what we see it ; it was once an egg, a semifluid mass 

 of yolk, not so big as a pin s head, contained in a transparent 

 membrane, and exhibiting not the least trace of any one of those 

 organs, whose multiplicity and complexity, in the adult, are so 



