204 THE STUDY OF ZOOLOGY vil 



what has been excessively developed and metamor- 

 phosed 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 in- 

 genuity of speculative anatomists proved itself 

 fully competent to spin any number of contradic- 

 tory hypotheses out of the same facts, and endless 

 morphological dreams threatened to supplant 

 scientific theory. 



Happily, however, there is a criterion of mor- 

 phological 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, the multiplicity and 

 complexity of which, 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. 



