100 LAY SERMONS, ADDRESSES, AND REVIEWS. [vi. 



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 moveable, 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 any- 

 thing 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 percep- 

 tion of the analogies which obtain between fully formed 

 parts. The unchecked ingenuity of speculative anato- 

 mists 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 morpho- 

 logical 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 exhi- 

 biting not the least 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 constric- 

 tions into segments, the forerunners of the rings of the 

 body. Upon the ventral surface of each of the rings 

 thus sketched out, a pair of bud-like prominences made 





