ON THE STUDY OF ZOOLOGY 357 



to see, that, in the legs, it is the part of the appendage which 

 corresponds with the inner division, which becomes modified 

 into what we know familiarly as the " leg," while the middle 

 division disappears, and the outer division is hidden under 

 the carapace. Nor is it more difficult to discern that, in the 

 appendages of the tail, the middle division appears again 

 and the outer vanishes ; while, on the other hand, in the fore- 

 most 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 appendage, 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 



